compositor: Use GL_BGRA_EXT for glReadPixels in screenshooter

This needs GL_EXT_read_format_bgra.

src/compositor.c

@@ -2321,6 +2321,11 @@ weston_compositor_init(struct weston_compositor *ec, struct wl_display *display)
 		return -1;
 	}
 
+	if (!strstr(extensions, "GL_EXT_read_format_bgra")) {
+		fprintf(stderr, "GL_EXT_read_format_bgra not available\n");
+		return -1;
+	}
+
 	extensions =
 		(const char *) eglQueryString(ec->display, EGL_EXTENSIONS);
 	if (strstr(extensions, "EGL_WL_bind_wayland_display"))

src/screenshooter.c

@@ -62,7 +62,7 @@ screenshooter_shoot(struct wl_client *client,
 
 	glPixelStorei(GL_PACK_ALIGNMENT, 1);
 	glReadPixels(0, 0, output->current->width, output->current->height,
-		     GL_RGBA, GL_UNSIGNED_BYTE, tmp);
+		     GL_BGRA_EXT, GL_UNSIGNED_BYTE, tmp);
 
 	d = wl_shm_buffer_get_data(buffer);
 	s = tmp + stride * (buffer->height - 1);