linux: Fix mouse cursor size and blur on Wayland (#21373)

Closes #15788, #13258

This is a long-standing issue with a few previous attempts to fix it,
such as [this one](https://github.com/zed-industries/zed/pull/17496).
However, that fix was later reverted because it resolved the blur issue
but caused a size issue. Currently, both blur and size issues persist
when you set a custom cursor size from GNOME Settings and use fractional
scaling.

This PR addresses both issues.

---

### Context

A new Wayland protocol,
[cursor-shape-v1](https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/194),
allows the compositor to handle rendering the cursor at the correct size
and shape. This protocol is implemented by KDE, wlroots (Sway-like
environments), etc. Zed supports this protocol, so there are no issues
on these desktop environments.

However, GNOME has not yet
[adopted](https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6212) this
protocol. As a result, apps must fall back to manually rendering the
cursor by specifying the theme, size, scale, etc., themselves. Zed also
implements this fallback but does not correctly account for the display
scale.

---

### Scale Fix

For example, if your cursor size is `64px` and you’re using fractional
scaling (e.g., `150%`), the display scale reported by the window query
will be an integer value, `2` in this case. Why `2` if the scale is
`150%`? That’s what the new protocol aims to improve. However, since
GNOME Wayland uses this integer scale everywhere, it’s sufficient for
our use case.

To fix the issue, we set the `buffer_scale` to this value. But that
alone doesn’t solve the problem. We also need to generate a matching
theme cursor size for this scaled version. This can be calculated as
`64px` * `2`, resulting in `128px` as the theme cursor size.

---

### Size Fix

The XDG Desktop Portal’s `cursor-size` event fails to read the cursor
size because it expects an `i32` but encounters a type error with `u32`.
Due to this, the cursor size was interpreted as the default `24px`
instead of the actual size set via user.

---

### Tested

This fix has been tested with all possible combinations of the
following:

- [x] GNOME Normal Scale (100%, 200%, etc.)
- [x] GNOME Fractional Scaling (125%, 150%, etc.)
- [x] GNOME Cursor Sizes (**Settings > Accessibility > Seeing**, e.g.,
`24px`, `64px`, etc.)
- [x] GNOME Experimental Feature `scale-monitor-framebuffer` (both
enabled and disabled)
- [x] KDE (`cursor-shape-v1` protocol)

---

**Result:**

64px custom cursor size + 150% Fractional Scale:


https://github.com/user-attachments/assets/cf3b1a0f-9a25-45d0-ab03-75059d3305e7

---

Release Notes:

- Fixed mouse cursor size and blur issues on Wayland

crates/gpui/src/platform/linux/wayland/window.rs

@@ -194,6 +194,23 @@ impl WaylandWindowState {
         self.decorations == WindowDecorations::Client
             || self.background_appearance != WindowBackgroundAppearance::Opaque
     }
+
+    pub fn primary_output_scale(&mut self) -> i32 {
+        let mut scale = 1;
+        let mut current_output = self.display.take();
+        for (id, output) in self.outputs.iter() {
+            if let Some((_, output_data)) = &current_output {
+                if output.scale > output_data.scale {
+                    current_output = Some((id.clone(), output.clone()));
+                }
+            } else {
+                current_output = Some((id.clone(), output.clone()));
+            }
+            scale = scale.max(output.scale);
+        }
+        self.display = current_output;
+        scale
+    }
 }
 
 pub(crate) struct WaylandWindow(pub WaylandWindowStatePtr);
@@ -560,7 +577,7 @@ impl WaylandWindowStatePtr {
 
                 state.outputs.insert(id, output.clone());
 
-                let scale = primary_output_scale(&mut state);
+                let scale = state.primary_output_scale();
 
                 // We use `PreferredBufferScale` instead to set the scale if it's available
                 if state.surface.version() < wl_surface::EVT_PREFERRED_BUFFER_SCALE_SINCE {
@@ -572,7 +589,7 @@ impl WaylandWindowStatePtr {
             wl_surface::Event::Leave { output } => {
                 state.outputs.remove(&output.id());
 
-                let scale = primary_output_scale(&mut state);
+                let scale = state.primary_output_scale();
 
                 // We use `PreferredBufferScale` instead to set the scale if it's available
                 if state.surface.version() < wl_surface::EVT_PREFERRED_BUFFER_SCALE_SINCE {
@@ -719,6 +736,10 @@ impl WaylandWindowStatePtr {
             (fun)()
         }
     }
+
+    pub fn primary_output_scale(&self) -> i32 {
+        self.state.borrow_mut().primary_output_scale()
+    }
 }
 
 fn extract_states<'a, S: TryFrom<u32> + 'a>(states: &'a [u8]) -> impl Iterator<Item = S> + 'a
@@ -732,23 +753,6 @@ where
         .flat_map(S::try_from)
 }
 
-fn primary_output_scale(state: &mut RefMut<WaylandWindowState>) -> i32 {
-    let mut scale = 1;
-    let mut current_output = state.display.take();
-    for (id, output) in state.outputs.iter() {
-        if let Some((_, output_data)) = &current_output {
-            if output.scale > output_data.scale {
-                current_output = Some((id.clone(), output.clone()));
-            }
-        } else {
-            current_output = Some((id.clone(), output.clone()));
-        }
-        scale = scale.max(output.scale);
-    }
-    state.display = current_output;
-    scale
-}
-
 impl rwh::HasWindowHandle for WaylandWindow {
     fn window_handle(&self) -> Result<rwh::WindowHandle<'_>, rwh::HandleError> {
         unimplemented!()