Texture-based rendering

[cxong]

After the SDL2 port #169, we can move to a more efficient rendering method, by rendering entire textures instead of software rendering into a surface. This should make the game much more efficient, especially on high resolutions.

I've already tested separating the menu background into a different texture and it works well. Contrary to what the SDL2 migration guide implies, rendering multiple textures, where one of them is a software-updated one, is perfectly fine.

Other advantages include:

• More efficient tile map / font rendering, by rendering the same tiles repeatedly using SDL_RenderCopy
• Replace BlitMasked, BlitTint et al with the more efficient SDL_SetTextureColorMod
• Semi-transparent textures, for better looking effects like muzzle flashes, explosion flames, text animations etc. Can also do glow effects, for rocket launcher, maybe some pseudo-lighting effects.
• Can efficiently scale the game graphics, to implement dynamic zoom (but may look bad because of the pixel graphics)

Some considerations:

• May need to introduce texture atlases to fully take advantage of tile map rendering. This conflicts a bit with the custom graphics functionality; C-Dogs may need to include its own texture packer.
• Need to re-implement brightness with a lower-quality alternative. There's SDL_SetWindowBrightness but from testing it affects the entire display (and can be overridden by apps like flux), and also cannot set brightness above 100%. The alternative, to overlay everything with a semi-transparent white texture, isn't going to look as good.

[cxong]

Most of this work has actually been done via #500 ; need to check if this can be closed off

Seems like there is a possible rendering bug in macOS, especially with two windows.

[cxong]

This has been mostly implemented (still need to do the HUD), however it's apparent that performance is worse, at least at low resolutions. I suspect it's due to texture switching, so this task should be put on hold for now.