Avoid switch-cases where default is a silent assert

[JayFoxRox]

This PR splits the pixel shader texturemode default: assert into the known unimplemented cases. This way we'll know which feature is broken by the assert location + we can also see which features we haven't done yet.

For other cases pcmaker has reported as problematic, I've added explicit messages to the default: assert.

I've also added some fallbacks / dummy code so the asserts can be skipped using -NDEBUG in some cases. However, this "feature" is merely a cruel hack and should be used with caution.

(This change also helps with automating the compatibility list as users only have to report the assert location. A script can then automatically fetch the lines in question from GitHub)

[JayFoxRox]

FIXME: assert proper stage indices for texture modes

Not today... this is far more complicated than it sounds like

[JayFoxRox]

Moved to espes