[Impeller] Make Metal layers readable (flutter#41085)

Resolves flutter/flutter#124612.

Doing this would also allow us to remove the final blit on iOS when
advanced blends/backdrop filters are present (all of the facilities
would remain in place for GLES + Vulkan though).

Author: bdero

shell/platform/darwin/ios/ios_surface_metal_impeller.mm

@@ -61,6 +61,10 @@
     layer.drawableSize = drawable_size;
   }
 
+  // Flutter needs to read from the color attachment in cases where there are effects such as
+  // backdrop filters. Flutter plugins that create platform views may also read from the layer.
+  layer.framebufferOnly = NO;
+
   // When there are platform views in the scene, the drawable needs to be presented in the same
   // transaction as the one created for platform views. When the drawable are being presented from
   // the raster thread, there is no such transaction.

shell/platform/darwin/ios/ios_surface_metal_skia.mm

@@ -55,7 +55,7 @@
 
   layer.pixelFormat = MTLPixelFormatBGRA8Unorm;
   // Flutter needs to read from the color attachment in cases where there are effects such as
-  // backdrop filters.
+  // backdrop filters. Flutter plugins that create platform views may also read from the layer.
   layer.framebufferOnly = NO;
 
   const auto drawable_size = CGSizeMake(frame_info.width(), frame_info.height());