draw: rework handling of non-existing outputs in emit code

Previously the code would just redirect requests for attributes which
don't exist to use output 0. Rework this to output all zeros instead which
seems more useful - in particular some extensions like
ARB_fragment_layer_viewport require 0 in the fs even if it wasn't output by
previous stages. That way, drivers don't have to special case this depending
if the vs/gs outputs some attribute or not.

Reviewed-by: Brian Paul <[email protected]>
Reviewed-by: Jose Fonseca <[email protected]>
Reviewed-by: Edward O'Callaghan <[email protected]>

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