render: optionally clear buffer before rendering

It's suboptimal to do it separately, and some drivers can do it for free this way
Prevents UNASSIGNED-BestPractices-vkCmdClearAttachments-clear-after-load warning

render/pixman/renderer.c

@@ -307,6 +307,13 @@ static struct wlr_render_pass *pixman_begin_buffer_pass(struct wlr_renderer *wlr
 	if (pass == NULL) {
 		return NULL;
 	}
+	if (options->clear_buffer) {
+		wlr_render_pass_add_rect(&pass->base, &(struct wlr_render_rect_options){
+			.box = { .width = buffer->buffer->width, .height = buffer->buffer->height },
+			.color = options->clear_color,
+			.blend_mode = WLR_RENDER_BLEND_MODE_NONE,
+		});
+	}
 	return &pass->base;
 }