Add shader uniform hints for screen textures so users can specify custom filter and repeat modes.

At this time, it works best in the Vulkan Renderers as they support using multiple samplers with the same texture.

In GLES3 this feature really only allows you to use the screen texture without mipmaps if you want to save the cost of generating them.

servers/rendering/shader_compiler.h

@@ -80,6 +80,7 @@ public:
 		bool uses_global_textures;
 		bool uses_fragment_time;
 		bool uses_vertex_time;
+		bool uses_screen_texture_mipmaps;
 	};
 
 	struct DefaultIdentifierActions {