mesa: fix PACK_COLOR_5551(), PACK_COLOR_1555() macros

mesa: fix PACK_COLOR_5551(), PACK_COLOR_1555() macros

The 1-bit alpha channel was incorrectly encoded.  Previously, any non-zero
alpha value for the ubyte alpha value would set A=1.  Instead, use the
most significant bit of the ubyte alpha to determine the A bit.  This is
consistent with the other channels and other OpenGL implementations.

Note: This is a candidate for the 7.11 branch.

Reviewed-by: Michel Dänzer <michel@daenzer.net>