output: Add function to set preferred render format

This change introduces new double buffered state to the wlr_output,
corresponding to the buffer format to render to.

The format being rendered to does not control the bit depth of colors
being sent to the display; it does generally determine the format with
which screenshot data is provided. The DRM backend _may_ sent higher
bit depths if the render format depth is increased, but hardware and
other limitations may apply.

include/wlr/interfaces/wlr_output.h

@@ -21,6 +21,7 @@
 	(WLR_OUTPUT_STATE_DAMAGE | \
 	WLR_OUTPUT_STATE_SCALE | \
 	WLR_OUTPUT_STATE_TRANSFORM | \
+	WLR_OUTPUT_STATE_RENDER_FORMAT | \
 	WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED)
 
 /**