Copy/paste images into editors (Mac only) (#15782)

For future reference: WIP branch of copy/pasting a mixture of images and
text: https://github.com/zed-industries/zed/tree/copy-paste-images -
we'll come back to that one after landing this one.

Release Notes:

- You can now paste images into the Assistant Panel to include them as
context. Currently works only on Mac, and with Anthropic models. Future
support is planned for more models, operating systems, and image
clipboard operations.

---------

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Jason <jason@zed.dev>
Co-authored-by: Kyle <kylek@zed.dev>

crates/gpui/src/geometry.rs

@@ -2447,10 +2447,24 @@ impl From<usize> for Pixels {
 /// affected by the device's scale factor, `DevicePixels` always correspond to real pixels on the
 /// display.
 #[derive(
-    Add, AddAssign, Clone, Copy, Default, Div, Eq, Hash, Ord, PartialEq, PartialOrd, Sub, SubAssign,
+    Add,
+    AddAssign,
+    Clone,
+    Copy,
+    Default,
+    Div,
+    Eq,
+    Hash,
+    Ord,
+    PartialEq,
+    PartialOrd,
+    Sub,
+    SubAssign,
+    Serialize,
+    Deserialize,
 )]
 #[repr(transparent)]
-pub struct DevicePixels(pub(crate) i32);
+pub struct DevicePixels(pub i32);
 
 impl DevicePixels {
     /// Converts the `DevicePixels` value to the number of bytes needed to represent it in memory.