mesa/st: add software-fallback for LATC

Here's the last piece of the puzzle for BC1-7 emulation in the mesa state-tracker, in line with !18180 (merged), !18012 (merged) and !18248 (merged).

I've split this out from !18248 (merged) (and it's based on top of it until that one lands), because the LATC emulation fails quite a few more tests than the RGTC emulation. LATC is also not a requirement for any specific GL version, so maybe we don't even need this.

This also rips out the fake_rgtc-support from u_transfer_helper while we're at it, because the functionality is no longer used.

Tested here