add render backend define

Author: darkskygit

src/capi/scdef.rs

@@ -74,6 +74,16 @@ pub enum GFX_LAYER
 
 	/// Skia backend with OpenGL rendering.
 	SKIA_OPENGL = 5,
+
+  SKIA_VULKAN = 6,
+
+  #[cfg(target_os = "macos")]
+  SKIA_METAL = 7,
+
+	/// A hardware Direct2D mode (Windows only).
+	#[cfg(windows)]
+	SKIA_DX12 = 8,
+
 }
 
 #[repr(C)]