mesa/get: Convert stencil values to TYPE_UINT.

mesa/get: Convert stencil values to TYPE_UINT.

These are listed as Z+ in the GL spec, and often have values of
0xFFFFFFFF.  For glGetFloat, we should return 4294967295.0 rather than
-1.0.  Similarly, for glGetInteger64v, we should return 0xFFFFFFFF, not
the sign extended 0xFFFFFFFFFFFFFFFF.

Fixes 6 dEQP tests matching the pattern
dEQP-GLES3.functional.state_query.integers.stencil*value*mask*getfloat
when run in a single process (with state reset code happening between
tests, which makes dEQP set the stencil value mask to 0xFFFFFFFF).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>