ngl: add front cache to reduce uniform changes

ngl: add front cache to reduce uniform changes

Since we make full snapshots when recording uniform state of batches, we
need to perform some deduplication to avoid so many repeated uniform calls.

This uses a closed hashtable to determine if we are likely changing the
value to something new.

This does not currently compare values, it instead only compares that we
are going to point at a new offset into the uniform buffer. We could go
further if we compare upon updating values (we did that early on in the
prototype) so that offsets are less likely to be changed.