Fix FP settings and turn off spam warning from clang 20.

[MarkCallow]

clang 20 has a new warning if one float-point option modifies a setting made by another. We are setting -ffp-model=precise which leaves contraction on and then -ffp-contract=off thus we get the warning. This usage is normal for cross-platform invariance thus this PR turns off the warning.

This warning was raised when compiling the ASTC encoder code but not when compiling the rest of the libktx code which was supposed to use the same settings. This PR also fixes the broken settings. The same FP options are now used everywhere.