Initial shot at a renderer target for Apple's Metal API.

This isn't complete, but is enough to run testsprite2. It's currently
Mac-only; with a little work to figure out how to properly glue in a Metal
layer to a UIView, this will likely work on iOS, too.

This is only wired up to the configure script right now, and disabled by
default. CMake and Xcode still need their bits filled in as appropriate.

src/render/SDL_render.c

@@ -92,6 +92,9 @@ static const SDL_RenderDriver *render_drivers[] = {
 #if SDL_VIDEO_RENDER_DIRECTFB
     &DirectFB_RenderDriver,
 #endif
+#if SDL_VIDEO_RENDER_METAL
+    &METAL_RenderDriver,
+#endif
 #if SDL_VIDEO_RENDER_PSP
     &PSP_RenderDriver,
 #endif