glrend - support shade tables

[vs49688]

Croc doesn't use shade tables, so they were never implemented in glrend. brtutor8b uses them, so now's the time.

[dethrace-labs]

We implemented it in the dethrace gl renderer which is similar to glrend (but was built before glrend was available).

Heres how we did it: https://github.com/dethrace-labs/dethrace/blob/main/src/harness/resources/3d_frag.glsl#L81-L93

[vs49688]

@dethrace-labs Is that code still in-use? I also see you've forked glrend, any chance of sync'ing relative changes?