Bug 1355183 - Mark WrExternalImageType as repr(u32) instead of repr(C). r=rhunt

On the C++ side the enum extends from uint32_t, like all the other enums across
the WR FFI boundary. So the Rust side they should be repr(u32).

MozReview-Commit-ID: JOf44I154ea

gfx/webrender_bindings/src/bindings.rs

@@ -440,7 +440,7 @@ impl From<ClipRegion> for WrClipRegion {
     }
 }
 
-#[repr(C)]
+#[repr(u32)]
 #[allow(dead_code)]
 enum WrExternalImageType {
     NativeTexture,