TrueColor: add ingame toggler

[JNechaevsky]

This adds simple ingame toggler for "TrueColor Rendering". Few remarks:

• It doesn't add back full revert to paletted renderer, so translucency effects are still expensive and no support for custom PLAYPAL tinting effects.
• I was forced to remove extra line break after subheaders to don't separate menus. From another hand, it gives some extra vertical space for easier adding possible new features.
• Might be not that useful for two builds, but probably will be handy for later.

[fabiangreffrath]

Looks good, thank you!

[JNechaevsky]

Before we'll merge this: I'm afraid we have bumped into a small misunderstanding about this switch with @rfomin. While my idea was to have just an in-game switch between paletted/truecolor views (not renderers), and that's what it do now, Roman was meaning full toggling.

But as already known, full toggling is a long and different story. Aside from possible turning pixel_t variables into a pointers and doubling R_Drawing functions for 8 and 32 bit, there will be one extra problem: upper level render functions have to be not just restarted, but instead fully destroyed and completely reinitialized again. I may be completely wrong here, though.

Another approach is to always have a TrueColor mode, but make it friendly with custom palette tints and take care about expensive translucency. In this case, only running on 16-bit and below display modes will not be possible.

Really no idea which way is better. First one is an absolute full mile and have to be more correct in technical terms, second one is probably simpler and shorter.

So, for now this PR doesn't do much, except adds some vertical space for Crispness menu.

[fabiangreffrath]

I think that's understood. It remains full truecolor rendering under the hood, but merely emulates paletted rendering by loading colormaps from the COLORMAP lump instead of fading RGB values to black. Translucency performance and custom palette issues still remain.

[JNechaevsky]

Yes, absolutely. You know this better than anyone, I know this after months of working with TrueColor code. But Roman wasn't working with TrueColor previously. 🙂

Still, should be merge this PR then? And what is your recommendation for farther compatibility support, i.e. which way we should move?

[rfomin]

Before we'll merge this: I'm afraid we have bumped into a small misunderstanding about this switch with @rfomin. While my idea was to have just an in-game switch between paletted/truecolor views (not renderers), and that's what it do now, Roman was meaning full toggling.

Yes, my idea was full toggling and remove the separate true color build. I think the current implementation of the toggle is not very useful.