mesa: Fix glGetUniform() type conversions.

mesa: Fix glGetUniform() type conversions.

We were primarily failing to convert in the NativeIntegers case, which
this fixes.  However, we were also just truncating float uniforms when
converting to integer, which does not appear to be the correct
behavior.  Note, however, that the NVIDIA drivers also truncate
instead of rounding.

GL_DOUBLE return type is dropped because it was never used and
completely broken.  It can be added when there's test code.

Fixes piglit ARB_shader_objects/getuniform

v2: This is a rewrite of my previous glGetUniform patch, which Ken
    pointed out missed storage_type-based conversions to integer,
    which was totally broken still thanks to a typo in the testcase.
v3: Quote the spec justifying the rounding behavior.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Ian Romanick <ian.d.romanick@intel.com>