[Impeller] fixed colorspace for metal screenshots (flutter#49941)

fixes flutter/flutter#141903

The golden tests were generating images in the wrong colorspace which
left things brighter than they should have been. This fixes that issue.

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impeller/golden_tests/metal_screenshotter.mm

@@ -37,8 +37,11 @@
     return {};
   }
 
-  CIImage* ciImage = [[CIImage alloc] initWithMTLTexture:metal_texture
-                                                 options:@{}];
+  CGColorSpaceRef color_space = CGColorSpaceCreateDeviceRGB();
+  CIImage* ciImage = [[CIImage alloc]
+      initWithMTLTexture:metal_texture
+                 options:@{kCIImageColorSpace : (__bridge id)color_space}];
+  CGColorSpaceRelease(color_space);
   FML_CHECK(ciImage);
 
   std::shared_ptr<Context> context = playground_->GetContext();