Add public APIs for creating a Metal view attached to an SDL window. Add SDL_metal.h.

include/SDL_config_iphoneos.h

@@ -181,6 +181,10 @@
 #define SDL_VIDEO_VULKAN 1
 #endif
 
+#if SDL_PLATFORM_SUPPORTS_METAL
+#define SDL_VIDEO_METAL 1
+#endif
+
 /* Enable system power support */
 #define SDL_POWER_UIKIT 1