Import SDL_gpu sources.

NOTE: no SCons build yet.
Enable OpenGL rendering.
Add alias for SDL_gpu/SDL_gpu/SDL_gpu.h.
Fix GPU_Init flags.
Port the load screen to use SDL_gpu.
Don't ignore CMakeLists.txt.
Commit missing CMakeLists for SDL_gpu.
Don't ignore txt files.
Fix SDL_gpu CMake build.

Patch provided by mordante.
Make SDL_gpu an optional dependency.
Remove SDL2-specific parts from loadscreen.
Add SDL_gpu implementation for fill_rect and draw_rect.
Remove SDL2 code from sdl/rect.
A function to create GPU_Rects easily.
SDL_gpu implementation for sdl::ttexture.
SDL_gpu implementation for get_texture et al.
SDL_gpu implementation for rendering background layers.
SDL_gpu implementation for rendering floating images.
SDL_gpu implementation for ttext::render_as_texture.
Remove SDL2 code.