gallium/util: load s3tc on demand
This changes the S3TC function pointers to be initialized to stubs that load the S3TC library and then delegate to the real functions. If the S3TC library fails to load, the function pointers are replaced with a "nop" function. The code is also changed to attempt to load the library only one time.c Note that unlike checking for a flag, this method has no performance cost at all. The use of the "nop" functions also allows to avoid most checks, that are only preserved when the function does non-trivial work.
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