glsl: check total count of multi-slot double vertex attribs

glsl: check total count of multi-slot double vertex attribs

The spec is vague all over the place about this, but this seems
to be the intent, we can probably make this optional later if
someone makes hw that cares and writes a driver.

Basically we need to double count some of the d types but
only for totalling not for slot number assignment.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Dave Airlie <airlied@redhat.com>