glfilter: only allow the same src/sink caps when we are in passthrough mode

glfilter: only allow the same src/sink caps when we are in passthrough mode

If we are given caps with extra features (like the overlay composition
features), we can only deal with that when we are in passthrough mode.

Previously we were bailing entirely and not allowing passthrough filter elements
with things like textoverlay.

Fixes the following pipeline (assuming glfilter supports passthrough):
gl ! textoverlay ! glfilter ! ... ! glimagesinkelement

https://bugzilla.gnome.org/show_bug.cgi?id=763756