glsl: correctly track cross slot component packing

glsl: correctly track cross slot component packing

Otherwise we will mix and match mesa's custom cross slot packing
with arb_enhanced_layouts style packing and we won't correctly
handle the size of the vars needed for the mesa custom packing.

The code was working correctly if the shader interface had both
a matching input and output but when we only had one side of
the interface we were only marking a single slot location as
packed.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Fixes: e5122a55435d ("glsl: add a NIR based varying linker")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6853
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17550>