mesa: Fix bug in unclamped float to ubyte conversion.

mesa: Fix bug in unclamped float to ubyte conversion.

Problem: The IEEE float optimized version of UNCLAMPED_FLOAT_TO_UBYTE
in macros.h computed incorrect results for inputs in the range
0x3f7f0000 (=0.99609375) to 0x3f7f7f80 (=0.99803924560546875)
inclusive.  0x3f7f7f80 is the IEEE float value that results in 254.5
when multiplied by 255.  With rounding mode "round to closest even
integer", this is the largest float in the range 0.0-1.0 that is
converted to 254 by the generic implementation of
UNCLAMPED_FLOAT_TO_UBYTE.  The IEEE float optimized version
incorrectly defined the cut-off for mapping to 255 as 0x3f7f0000
(=255.0/256.0). The same bug was present in the function
float_to_ubyte in u_math.h.

Fix: The proposed fix replaces the incorrect cut-off value by
0x3f800000, which is the IEEE float representation of 1.0f. 0x3f7f7f81
(or any value in between) would also work, but 1.0f is probably
cleaner.

The patch does not regress piglit on llvmpipe and on i965 on sandy
bridge.

Tested-by Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Brian Paul <brianp@vmware.com>