Fix #2249: macOS converts the canvas from sRGB to the display's profile

The window's CAMetalLayer had no color space: wgpu 30.0.1 sets it to
nil for an sRGB surface each time it configures the surface, and Core
Animation does not color-match a layer without one. On a wide-gamut
display #FF0000 showed as the display's own red, (255, 0, 0) in a
capture where Preview shows the same sRGB red as (234, 51, 35).

A paint callback that draws nothing runs after egui configures the
surface and before the frame is presented. It sets sRGB on each
window's Metal layer that has no color space and leaves any other color
space as it is. wgpu's own fix (gfx-rs/wgpu#10286) is in no release yet.

Fixes #3161.

Cargo.lock

@@ -3282,6 +3282,7 @@ dependencies = [
  "objc2-core-foundation",
  "objc2-core-graphics",
  "objc2-foundation 0.3.2",
+ "objc2-quartz-core 0.3.2",
 ]
 
 [[package]]
@@ -6052,8 +6053,10 @@ dependencies = [
  "objc2 0.6.5",
  "objc2-app-kit 0.3.2",
  "objc2-core-foundation",
+ "objc2-core-graphics",
  "objc2-core-text",
  "objc2-foundation 0.3.2",
+ "objc2-quartz-core 0.3.2",
  "pollster 1.0.1",
  "raw-window-handle",
  "rfd",