The blend math gets a bit funky due to inverse blend factors being
in range [0,2] rather than [-1,1], our normalized math can't really
cover this.
src_alpha_saturate blend factor has a similar problem too.
(Note that piglit fbo-blending-formats test is mostly useless for
anything but unorm formats, since not just all src/dst values are
between [0,1], but the tests are crafted in a way that the results
are between [0,1] too.)

v2: some formatting fixes, and fix a fairly obscure (to debug)
issue with alpha-only formats (not related to snorm at all), where
blend optimization would think it could simplify the blend equation
if the blend factors were complementary, however was using the
completely unrelated rgb blend factors instead of the alpha ones...

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