[Impeller] switches the wide gamut surface to f16 (flutter/engine#41994)

**requires https://github.com/flutter/flutter/pull/126712 to land
first**

fixes https://github.com/flutter/flutter/issues/126620
integration test at: https://github.com/flutter/flutter/pull/126715

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above.
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engine/src/flutter/shell/gpu/gpu_surface_metal_impeller.mm

@@ -208,6 +208,10 @@ Surface::SurfaceData GPUSurfaceMetalImpeller::GetSurfaceData() const {
       bytesPerPixel = 4;
       pixel_format = "MTLPixelFormatBGRA8Unorm";
       break;
+    case MTLPixelFormatRGBA16Float:
+      bytesPerPixel = 8;
+      pixel_format = "MTLPixelFormatRGBA16Float";
+      break;
     default:
       return {};
   }

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterView.mm

@@ -107,7 +107,11 @@
       CGColorSpaceRef srgb = CGColorSpaceCreateWithName(kCGColorSpaceExtendedSRGB);
       layer.colorspace = srgb;
       CFRelease(srgb);
-      layer.pixelFormat = MTLPixelFormatBGRA10_XR;
+      // MTLPixelFormatRGBA16Float was chosen since it is compatible with
+      // impeller's offscreen buffers which need to have transparency.  Also,
+      // F16 was chosen over BGRA10_XR since Skia does not support decoding
+      // BGRA10_XR.
+      layer.pixelFormat = MTLPixelFormatRGBA16Float;
     }
   }