config/output: Add support for 6-bit render fmt

GUD devices uses RGB565 by default for performance reasons. Allow
specifying render_bit_depth 6 to pick this format. The definition works
out if you consider the maximum number of bits per channel instead of
the average.

include/sway/config.h

@@ -262,6 +262,7 @@ enum scale_filter_mode {
 
 enum render_bit_depth {
 	RENDER_BIT_DEPTH_DEFAULT, // the default is currently 8
+	RENDER_BIT_DEPTH_6,
 	RENDER_BIT_DEPTH_8,
 	RENDER_BIT_DEPTH_10,
 };