render/texture: add width and height fields

Instead of requiring compositors to call wlr_texture_get_size each time
they want to access the texture's size, expose this information as
wlr_texture fields.

include/render/gles2.h

@@ -82,7 +82,6 @@ struct wlr_gles2_texture {
 
 	EGLImageKHR image;
 
-	int width, height;
 	bool inverted_y;
 	bool has_alpha;