screenshoter: don't use hardcoded bpp value

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
diff --git a/src/screenshooter.c b/src/screenshooter.c
index 840dfb8..662aad4 100644
--- a/src/screenshooter.c
+++ b/src/screenshooter.c
@@ -103,7 +103,7 @@
 
 	output->disable_planes--;
 	wl_list_remove(&listener->link);
-	stride = l->buffer->width * 4;
+	stride = l->buffer->width * (PIXMAN_FORMAT_BPP(output->compositor->read_format) / 8);
 	pixels = malloc(stride * l->buffer->height);
 
 	if (pixels == NULL) {