mesa: fix possible precision issues in pack/unpack/fetch functions

mesa: fix possible precision issues in pack/unpack/fetch functions

GLfloat doesn't have enough precision to exactly represent 0xffffff
and 0xffffffff. (and a reciprocal of those, if I am not mistaken)

If -ffast-math is enabled, using GLfloat causes assertion failures in:
- fbo-blit-d24s8
- fbo-depth-sample-compare
- fbo-readpixels-depth-formats
- glean/depthStencil

For example:
fbo-depth-sample-compare: main/format_unpack.c:1769:
unpack_float_z_Z24_X8: Assertion `dst[i] <= 1.0F' failed.

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