gl-renderer: fix performance regression in frag

When color management is disabled, the fragment shader was still first ensuring straight alpha and then immediately just going back to pre-multiplied. This is near-impossible for a shader compiler to optimize out, I guess because of the if-statement to handle division by zero. Having view alpha applied in between certainly didn't make it easier.

That causes extra fragment computations that are unnecessary. In the issue report this was found to cause a notable performance regression.

Fix the performance regression by introducing special-case paths for when straight alpha is not needed. This skips the unnecessary computations.

Fixes: #623 (closed)

Fixes: 9a6a4e70