glsl: relax rule on varying matching for shaders older than 4.00
Please see new code commment for full justification. Fixes: 18004c33 ("glsl: fail when a shader's input var has not an equivalent out var in previous") Closes: mesa/mesa#3648 Reviewed-by:Matt Turner <mattst88@gmail.com> Reviewed-by:
Samuel Iglesias Gonsálvez <siglesias@igalia.com> Part-of: <mesa/mesa!7184>