llvmpipe: first steps of adding dual source blend support

llvmpipe: first steps of adding dual source blend support

This adds support of the additional blending factors to the blend function
itself, and also enables testing of it in lp_test_blend (which passes).
Still need to add the glue code of linking fs shader outputs to blend inputs
in llvmpipe, and probably need to add special handling if destination doesn't
include alpha (which lp_test_blend doesn't test).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>