Fix alpha zero pixels wreaking havoc
One place accidentally ignored alpha-zero pixels, making it inconsistent
with the rest and causing bad visual errors in MODE_FAST.

Origin: upstream, https://github.com/divVerent/s2tc/commit/1fc9f34d7c81a43c5bebc2730d3636116574ee03
Fix a weird bug in selecting initial colors by wrong order of loads/stores.
Origin: upstream, https://github.com/divVerent/s2tc/commit/239cf3356452cd7fe920b4ff7bfbef9e3a0dbe29
reenable alpha 0 pixel ignorance for DXT1
As DXT1 never stores color values where alpha is 0.

Origin: upstream, https://github.com/divVerent/s2tc/commit/8d4ff5b2c897a2747267c79f8feb728e81b71886
Reset s2tc refinement to faster default value

The alpha pixel bleeding effect was fixed in the last three commits and
therefore the faster refinement method can be used again.
Fix uninitialized memory access in s2tc
Origin: upstream, https://github.com/divVerent/s2tc/commit/c9a70d0f45cfd4d564987a8c8761f4c7ce1a39ab
Reduce diff to upstream S2TC by just not compiling env config code
Don't include thread headers when filter threads are disabled