mesa: Set gl_constants::MinMapBufferAlignment

Leaving it set to zero isn't really correct since every allocation has
at least an alignment of 1 byte.  It also caused a problem in the i965
driver after I removed the MAX(64, ...) from the alignment calculation.
That's what I get for changing a patch without retesting it. :(

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73907
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: Lu Hua <huax.lu@intel.com>

src/mesa/main/context.c

@@ -587,6 +587,7 @@ _mesa_init_constants(struct gl_context *ctx)
    ctx->Const.MaxSpotExponent = 128.0;
    ctx->Const.MaxViewportWidth = MAX_VIEWPORT_WIDTH;
    ctx->Const.MaxViewportHeight = MAX_VIEWPORT_HEIGHT;
+   ctx->Const.MinMapBufferAlignment = 1;
 
    /* Driver must override these values if ARB_viewport_array is supported. */
    ctx->Const.MaxViewports = 1;