mesa: Make the gl_constant_value's bool occupy the same space as float/int.

mesa: Make the gl_constant_value's bool occupy the same space as float/int.

At least for Intel, all our uniform components are of uint32_t size, either
float or signed or unsigned int.  For uploading uniform data in the driver,
it's much easier to upload a full dword per uniform element instead of trying
to pick out the bool byte and then fill in the top 3 bytes of pad with 0.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>