gpui: Add drop_image (#19772)

This PR adds a function, WindowContext::drop_image, to manually remove a
RenderImage from the sprite atlas. In addition, PlatformAtlas::remove
was added to support this behavior. Previously, there was no way to
request a RenderImage to be removed from the sprite atlas, and since
they are not removed automatically the sprite would remain in video
memory once added until the window was closed. This PR allows a
developer to request the image be dropped from memory manually, however
it does not add automatic removal.

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/gpui/src/platform.rs

@@ -46,6 +46,7 @@ use smallvec::SmallVec;
 use std::borrow::Cow;
 use std::hash::{Hash, Hasher};
 use std::io::Cursor;
+use std::ops;
 use std::time::{Duration, Instant};
 use std::{
     fmt::{self, Debug},
@@ -561,6 +562,42 @@ pub(crate) trait PlatformAtlas: Send + Sync {
         key: &AtlasKey,
         build: &mut dyn FnMut() -> Result<Option<(Size<DevicePixels>, Cow<'a, [u8]>)>>,
     ) -> Result<Option<AtlasTile>>;
+    fn remove(&self, key: &AtlasKey);
+}
+
+struct AtlasTextureList<T> {
+    textures: Vec<Option<T>>,
+    free_list: Vec<usize>,
+}
+
+impl<T> Default for AtlasTextureList<T> {
+    fn default() -> Self {
+        Self {
+            textures: Vec::default(),
+            free_list: Vec::default(),
+        }
+    }
+}
+
+impl<T> ops::Index<usize> for AtlasTextureList<T> {
+    type Output = Option<T>;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        &self.textures[index]
+    }
+}
+
+impl<T> AtlasTextureList<T> {
+    #[allow(unused)]
+    fn drain(&mut self) -> std::vec::Drain<Option<T>> {
+        self.free_list.clear();
+        self.textures.drain(..)
+    }
+
+    #[allow(dead_code)]
+    fn iter_mut(&mut self) -> impl DoubleEndedIterator<Item = &mut T> {
+        self.textures.iter_mut().flatten()
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]