mesa: add a UniformBooleanTrue option

Drivers supporting native integers set UniformBooleanTrue to the integer value
that should be used for true when uploading uniform booleans.  This is ~0 for
Gallium and 1 for i965.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/state_tracker/st_extensions.c

@@ -207,6 +207,7 @@ void st_init_limits(struct st_context *st)
       c->MaxProgramTexelOffset = screen->get_param(screen, PIPE_CAP_MAX_TEXEL_OFFSET);
 
       c->GLSLVersion = 120;
+      c->UniformBooleanTrue = ~0;
    }
 }