render: ensure wlr_render_rect_options->box is nonempty

This optimization also fixes an validation error with the Vulkan
renderer by ensuring vkCmdClearAttachments does not receive empty
regions.

include/wlr/render/interface.h

@@ -76,6 +76,7 @@ struct wlr_render_pass_impl {
 	bool (*submit)(struct wlr_render_pass *pass);
 	void (*add_texture)(struct wlr_render_pass *pass,
 		const struct wlr_render_texture_options *options);
+	/* Implementers are also guaranteed that options->box is nonempty */
 	void (*add_rect)(struct wlr_render_pass *pass,
 		const struct wlr_render_rect_options *options);
 };

render/pass.c

@@ -40,6 +40,9 @@ void wlr_render_pass_add_texture(struct wlr_render_pass *render_pass,
 
 void wlr_render_pass_add_rect(struct wlr_render_pass *render_pass,
 		const struct wlr_render_rect_options *options) {
+	if (wlr_box_empty(&options->box)) {
+		return;
+	}
 	render_pass->impl->add_rect(render_pass, options);
 }