iOS: Eliminate unnecessary bridged retain casts (flutter/engine#56662)

This replaces a few ARC bridged retain casts to regular bridge casts to CoreFoundation types, which are then CFRetained via `sk_cfp::retain` calls.

This eliminates the last remaining unnecessary __bridge_retained casts in the codebase. The remaining casts have been manually vetted and are reasonable.

Issue: flutter#155943

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: cbracken

engine/src/flutter/shell/platform/darwin/graphics/FlutterDarwinExternalTextureMetal.mm

@@ -277,14 +277,14 @@ @implementation FlutterDarwinExternalTextureSkImageWrapper
   return nullptr;
 #else   // SLIMPELLER
   GrMtlTextureInfo ySkiaTextureInfo;
-  ySkiaTextureInfo.fTexture = sk_cfp<const void*>{(__bridge_retained const void*)yTex};
+  ySkiaTextureInfo.fTexture.retain((__bridge GrMTLHandle)yTex);
 
   GrBackendTexture skiaBackendTextures[2];
   skiaBackendTextures[0] =
       GrBackendTextures::MakeMtl(width, height, skgpu::Mipmapped::kNo, ySkiaTextureInfo);
 
   GrMtlTextureInfo uvSkiaTextureInfo;
-  uvSkiaTextureInfo.fTexture = sk_cfp<const void*>{(__bridge_retained const void*)uvTex};
+  uvSkiaTextureInfo.fTexture.retain((__bridge GrMTLHandle)uvTex);
 
   skiaBackendTextures[1] =
       GrBackendTextures::MakeMtl(width, height, skgpu::Mipmapped::kNo, uvSkiaTextureInfo);
@@ -308,7 +308,7 @@ GrYUVABackendTextures yuvaBackendTextures(yuvaInfo, skiaBackendTextures,
 #else   // SLIMPELLER
 
   GrMtlTextureInfo skiaTextureInfo;
-  skiaTextureInfo.fTexture = sk_cfp<const void*>{(__bridge_retained const void*)rgbaTex};
+  skiaTextureInfo.fTexture.retain((__bridge GrMTLHandle)rgbaTex);
 
   GrBackendTexture skiaBackendTexture =
       GrBackendTextures::MakeMtl(width, height, skgpu::Mipmapped ::kNo, skiaTextureInfo);