Migrate rendering to SDL Shader API

I put together a [graphics rendering roadmap](https://github.com/gemrb/gemrb/wiki/Graphic-Rendering-Roadmap) a while ago detailing how we can solve some of the ongoing issues/performance/complexity with rendering.

Since then SDL has been [awarded a grant](https://www.patreon.com/posts/new-project-top-58563886) from Epic games to develop shader capabilities for `SDL_Renderer`. This is _huge_ news for us.

This would truly enable us to fix all the issues I mentioned in the roadmap without having to force OpenGL on people. We wouldn't have any direct dependency on OpenGL for these benefits, nor would we rely on hacks to control the active shader.