renderer: replace get_render_buffer_caps() with struct field

include/wlr/render/wlr_renderer.h

@@ -26,6 +26,10 @@ struct wlr_fbox;
  * A renderer for basic 2D operations.
  */
 struct wlr_renderer {
+	// Capabilities required for the buffer used as a render target (bitmask of
+	// enum wlr_buffer_cap)
+	uint32_t render_buffer_caps;
+
 	struct {
 		struct wl_signal destroy;
 		/**