screenshooter: Use the renderer function for reading out pixels

This also changes the compositor's read_format to a pixman format.
diff --git a/src/compositor.h b/src/compositor.h
index e2dbd0d..57d363a 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -353,7 +353,7 @@
 	PFNEGLDESTROYIMAGEKHRPROC destroy_image;
 
 	int has_unpack_subimage;
-	GLenum read_format;
+	pixman_format_code_t read_format;
 
 	PFNEGLBINDWAYLANDDISPLAYWL bind_display;
 	PFNEGLUNBINDWAYLANDDISPLAYWL unbind_display;