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>
(cherry picked from commit 4731a598f0)

src/mesa/main/colormac.h

@@ -188,11 +188,11 @@ do {						\
 
 #define PACK_COLOR_5551( R, G, B, A )					\
    ((((R) & 0xf8) << 8) | (((G) & 0xf8) << 3) | (((B) & 0xf8) >> 2) |	\
-    ((A) ? 1 : 0))
+    ((A) >> 7))
 
 #define PACK_COLOR_1555( A, B, G, R )					\
    ((((B) & 0xf8) << 7) | (((G) & 0xf8) << 2) | (((R) & 0xf8) >> 3) |	\
-    ((A) ? 0x8000 : 0))
+    (((A) & 0x80) << 8))
 
 #define PACK_COLOR_1555_REV( A, B, G, R )					\
    ((((B) & 0xf8) >> 1) | (((G) & 0xc0) >> 6) | (((G) & 0x38) << 10) | (((R) & 0xf8) << 5) |	\