implement buffer scale

Keep track of the scale of the buffer set by surface.set_buffer_scale.

Calculate the height and width of the texture from the buffer depending on the
scale when it is attached.

Use buffer damage to determine the damage of the buffer when flushing damage.

Convert surface damage to buffer damage and union to the surface buffer_damage
prior to flushing damage.

include/wlr/render.h

@@ -61,6 +61,7 @@ struct wlr_texture {
 	bool valid;
 	uint32_t format;
 	int width, height;
+	int height_from_buffer, width_from_buffer;
 	struct wl_signal destroy_signal;
 	struct wl_resource *resource;
 };