output: use XRGB8888 format instead of ARGB8888

Most (and possibly all) compositors using wlroots only ever render
fully opaque content. To provide better performance, this change
switches the default format used by wlr_output buffers from
ARGB8888 to the opaque XRGB8888.

Compositors like mutter, kwin, and weston already default to
XRGB8888, so this change is unlikely to expose any new bugs in
underlying drivers and hardware.

This does not affect the hardware cursor's buffer format, which is
still ARGB8888 by default.

As part of this change, the X11 backend (which does not support
changing format at runtime) now picks a true color, 24 bit depth
visual (i.e. XRGB8888) instead of a 32 bit depth (ARGB8888) one.

backend/x11/backend.c

@@ -572,9 +572,9 @@ struct wlr_backend *wlr_x11_backend_create(struct wl_display *display,
 		goto error_event;
 	}
 
-	x11->depth = get_depth(x11->screen, 32);
+	x11->depth = get_depth(x11->screen, 24);
 	if (!x11->depth) {
-		wlr_log(WLR_ERROR, "Failed to get 32-bit depth for X11 screen");
+		wlr_log(WLR_ERROR, "Failed to get 24-bit depth for X11 screen");
 		goto error_event;
 	}
 

types/output/render.c

@@ -67,11 +67,7 @@ static bool output_create_swapchain(struct wlr_output *output,
 	}
 
 	struct wlr_drm_format *format = output_pick_format(output, display_formats,
-		DRM_FORMAT_ARGB8888);
-	if (format == NULL) {
-		format = output_pick_format(output, display_formats,
 		DRM_FORMAT_XRGB8888);
-	}
 	if (format == NULL) {
 		wlr_log(WLR_ERROR, "Failed to pick primary buffer format for output '%s'",
 			output->name);