glupload: Only offer custom allocator with caps features

glupload: Only offer custom allocator with caps features

To use GLMemory and EGLImage allocators, one need to know the
libgstgl API. This is only expected if the associated caps features
have been negotiated. Generic element that otherwise receive those
allocators may fail, resulting in broken pieline. We don't want to
force all generic element to check if the allocator is a custom
allocator or a normal allocator (which implement the _alloc method).

https://bugzilla.gnome.org/show_bug.cgi?id=758877