mesa: Don't round-trip integer texture data through a floating point temp.

mesa: Don't round-trip integer texture data through a floating point temp.

This was losing bits of precision.  Fixes (with the previous commits):
piglit EXT_texture_integer/getteximage-clamping
piglit EXT_texture_integer/getteximage-clamping GL_ARB_texture_rg
oglc advanced.mipmap.upload

Regresses oglc negative.typeFormatMismatch.teximage from fail to
abort, because it's been hitting texstore for a format/type combo that
shouldn't happen.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Brian Paul <brianp@vmware.com>