Fix #34156: Spec. and Alpha Intensity OpenGL issue

Issue was caused by alpha pipeline cleanup: apparently depending on
use_alpha flag different channels for spec/alpha would be used.

Made it so talpha is computed from Image->ignore_alpha instead of
always considering to be TRUTH.

This is not so much trivial to understand what's going on here, but
it's not new issue. Anyway, if someone have got ideas how to improve
feedback here -- ideas are welcome! For now only regression is fixed.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54374 954f8c5b-7b00-dc11-b283-0030488c597c