gpui: Add support for window transparency & blur on macOS (#9610)

This PR adds support for transparent and blurred window backgrounds on
macOS.

Release Notes:

- Added support for transparent and blurred window backgrounds on macOS
([#5040](https://github.com/zed-industries/zed/issues/5040)).
- This requires themes to specify a new `background.appearance` key
("opaque", "transparent" or "blurred") and to include an alpha value in
colors that should be transparent.

<img width="913" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/7547ee2a-e376-4d55-9114-e6fc2f5110bc">
<img width="994" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/b36fbc14-6e4d-4140-9448-69cad803c45a">
<img width="1020" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/d70e2005-54fd-4991-a211-ed484ccf26ef">

---------

Co-authored-by: Luiz Marcondes <luizgustavodevergennes@gmail.com>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/gpui/src/platform.rs

@@ -190,6 +190,7 @@ pub(crate) trait PlatformWindow: HasWindowHandle + HasDisplayHandle {
     fn activate(&self);
     fn is_active(&self) -> bool;
     fn set_title(&mut self, title: &str);
+    fn set_background_appearance(&mut self, background_appearance: WindowBackgroundAppearance);
     fn set_edited(&mut self, edited: bool);
     fn show_character_palette(&self);
     fn minimize(&self);
@@ -533,6 +534,9 @@ pub struct WindowOptions {
     /// The display to create the window on, if this is None,
     /// the window will be created on the main display
     pub display_id: Option<DisplayId>,
+
+    /// The appearance of the window background.
+    pub window_background: WindowBackgroundAppearance,
 }
 
 /// The variables that can be configured when creating a new window
@@ -555,6 +559,8 @@ pub(crate) struct WindowParams {
     pub show: bool,
 
     pub display_id: Option<DisplayId>,
+
+    pub window_background: WindowBackgroundAppearance,
 }
 
 impl Default for WindowOptions {
@@ -572,6 +578,7 @@ impl Default for WindowOptions {
             is_movable: true,
             display_id: None,
             fullscreen: false,
+            window_background: WindowBackgroundAppearance::default(),
         }
     }
 }
@@ -633,6 +640,27 @@ impl Default for WindowAppearance {
     }
 }
 
+/// The appearance of the background of the window itself, when there is
+/// no content or the content is transparent.
+#[derive(Copy, Clone, Debug, Default, PartialEq)]
+pub enum WindowBackgroundAppearance {
+    /// Opaque.
+    ///
+    /// This lets the window manager know that content behind this
+    /// window does not need to be drawn.
+    ///
+    /// Actual color depends on the system and themes should define a fully
+    /// opaque background color instead.
+    #[default]
+    Opaque,
+    /// Plain alpha transparency.
+    Transparent,
+    /// Transparency, but the contents behind the window are blurred.
+    ///
+    /// Not always supported.
+    Blurred,
+}
+
 /// The options that can be configured for a file dialog prompt
 #[derive(Copy, Clone, Debug)]
 pub struct PathPromptOptions {