compositor: use GL_EXT_unpack_subimage only if available

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/src/compositor.h b/src/compositor.h
index a27a7a0..e381e66 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -221,6 +221,9 @@
 	PFNGLEGLIMAGETARGETTEXTURE2DOESPROC image_target_texture_2d;
 	PFNEGLCREATEIMAGEKHRPROC create_image;
 	PFNEGLDESTROYIMAGEKHRPROC destroy_image;
+
+	int has_unpack_subimage;
+
 	PFNEGLBINDWAYLANDDISPLAYWL bind_display;
 	PFNEGLUNBINDWAYLANDDISPLAYWL unbind_display;
 	int has_bind_display;