mesa: Pass gl_constant_value union into _mesa_fetch_state().

mesa: Pass gl_constant_value union into _mesa_fetch_state().

We've had some trouble in the past with copying integers around via
float pointers, as the C compiler sometimes uses x87 floating point
registers to load values on 32-bit systems.  Passing the
gl_constant_value union should be safer.

To avoid churn, this patch creates a "GLfloat *value" variable so
existing uses can stay the same.

Not observed to fix anything, but I was in the area adding more integer
state vars, and thought it'd be wise.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Cc: mesa-stable@lists.freedesktop.org