anchor.diff:  * 'SDL_mutex.h'
   * 'SDL_quit.h' (only necessary when using `SDL_Main`?)
   * 'SDL_thread.h'
  +* 'SDL_video.h': the functions `SDL_CreateWindowFrom`, `SDL_SetWindowData` and `SDL_GetWindowData` take external data as argument.
   
   == Nothing to implement
   
  
positive.code: ** `SDL_GetRenderTarget` (renderer)
  ** `SDL_RenderIsClipEnabled` (renderer)
  ** `SDL_RenderReadPixels` (renderer)
  ** `SDL_GL_BindTexture` (gl)
  ** `SDL_GL_UnbindTexture` (gl)
  * 'SDL_stdinc.h': SDL_bool is implemented in 'sdl_bool.c'. Do we need anything else?
  * 'SDL_surface.h': Only surface creation (via `IMG_Load`) and destruction is implemented. Might be better to move IMG_* functions in their own space.
  * 'SDL_version.h': `SDL_GetRevisionNumber` must be implemented. The macros may also be useful.
  * 'SDL_video.h': The following elements are missing:
  ** `SDL_WINDOWPOS_*` values for different displays
  ** `SDL_CreateWindowFrom` (unclear if we need it)
  ** We currently do not support UTF-8. We should probably switch to binaries as input/output to support it for `SDL_SetWindowTitle` and `SDL_GetWindowTitle`
  ** `SDL_SetWindowData` (unclear if we need it)
  ** `SDL_GetWindowData` (unclear if we need it)
  ** `SDL_GetWindowSurface` (window)
  ** `SDL_UpdateWindowSurface` (window)
  ** `SDL_UpdateWindowSurfaceRects` (window)
  ** `SDL_GL_LoadLibrary` (unclear if we need it)
  ** `SDL_GL_GetProcAddress` (unclear if we need it)
  ** `SDL_GL_UnloadLibrary` (unclear if we need it)
  
negative.code: ** `SDL_GL_GetDrawableSize`
  ** `SDL_GL_SetSwapInterval`
  ** `SDL_GL_GetSwapInterval`
  * 'SDL_vulkan.h'
  
  == To be removed
  
  