crt-lottes and pixellate now use hardware linearization

[johnnovak]

Hey @tyrells, we have added support for sRGB textures and framebuffers for GLSL shaders in DOSBox Staging (see here). Shaders can now request these features via pragmas:

#pragma use_srgb_texture
#pragma use_srgb_framebuffer

The nice thing about this is that it's completely backwards compatible -- the shader compiler will always ignore these unknown pragmas, it's a pre-processor step in DOSBox Staging that looks for them.

We are definitely planning to merge this feature back upstream into the DOSBox SVN branch.

This PR updates all shaders in your repo that benefit from proper gamma-correct interpolation. I've tried to add it to all shaders that don't do their own linearisation (like most CRT shaders), but sadly they seem to be tuned for incorrect "sRGB space" interpolation so they look quite different and basically broken when things are done in the mathematically correct way. Because of this, I've just left them alone.

If you're interested, here are our discussions with before/after screenshots about why we introduced this feature in the first place, but I'm sure you're aware of all the reasons:

dosbox-staging/dosbox-staging#1627
dosbox-staging/dosbox-staging#1634

[johnnovak]

Hey @tyrells, just another ping, would you be able to merge this? Thanks.