Implement wrapping IOSurfaces in dawn::Texture.

This allows wrapping IOSurfaces in a dawn::Texture so a Dawn application
can sample from, or render to an IOSurface. It uses Metal's
functionality for wrapping textures in MTLTexture.

Support for single-plane BGRA8, RG8 and R8 IOSurfaces is added as well
as tests for sampling and using BeginRenderPass to clear them.

BUG=dawn:112

Change-Id: I367dbd1a75a0c7b81901fb0aae05f1cd46af3f3a
Reviewed-on: https://dawn-review.googlesource.com/c/5101
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/include/dawn_native/MetalBackend.h

@@ -18,10 +18,24 @@
 #include <dawn/dawn_wsi.h>
 #include <dawn_native/DawnNative.h>
 
-#import <Metal/Metal.h>
+struct __IOSurface;
+typedef __IOSurface* IOSurfaceRef;
+
+#ifdef __OBJC__
+#    import <Metal/Metal.h>
+#endif  //__OBJC__
 
 namespace dawn_native { namespace metal {
-    DAWN_NATIVE_EXPORT id<MTLDevice> GetMetalDevice(dawnDevice device);
+    DAWN_NATIVE_EXPORT dawnTexture WrapIOSurface(dawnDevice device,
+                                                 const dawnTextureDescriptor* descriptor,
+                                                 IOSurfaceRef ioSurface,
+                                                 uint32_t plane);
 }}  // namespace dawn_native::metal
 
+#ifdef __OBJC__
+namespace dawn_native { namespace metal {
+    DAWN_NATIVE_EXPORT id<MTLDevice> GetMetalDevice(dawnDevice device);
+}}      // namespace dawn_native::metal
+#endif  // __OBJC__
+
 #endif  // DAWNNATIVE_METALBACKEND_H_