mesa: Allocate tokens from the heap.

mesa: Allocate tokens from the heap.

The recent increase ST_MAX_SHADER_TOKENS to 8K causes stack overflows on
windows.

Failure to allocate is not being propagated to the caller. This is not
a regression since the previous _mesa_malloc result wasn't being
checked as well. Unfortunately it is not easy to fix, as the callers of
these functions do not have failure propagation mechanism either, and
so on. So leaving a just fixme note for now.