anchor.diff:  ** `SDL_GL_GetDrawableSize`
   ** `SDL_GL_SetSwapInterval`
   ** `SDL_GL_GetSwapInterval`
  -
  -The code sometimes uses `malloc`. Use `enif_alloc` everywhere instead.
  -
  -== To be implemented
  -
  -* 'SDL_audio.h'
  -* 'SDL_error.h' (for completion)
  -* 'SDL_gamecontroller.h'
  -* 'SDL_gesture.h'
  -* 'SDL_haptic.h'
  -* 'SDL_joystick.h'
  -* 'SDL_messagebox.h'
  -* 'SDL_rwops.h' (unclear if we need it)
  -* 'SDL_shape.h'
  -* 'SDL_system.h'
  -* 'SDL_syswm.h'
  -* 'SDL_timer.h' (unclear if we need it)
  -* 'SDL_touch.h'
   * 'SDL_vulkan.h'
  -
  -For OpenGL we need to figure out whether we can call the functions from
  -wxErlang. If we can, great! If not, find an automated way to provide
  -access to OpenGL.
  -
  -SDL extensions also need to be investigated and implemented.
  -We definitely want at least some of SDL_image, SDL_mixer
  -and SDL_ttf. We probably do not need SDL_net or SDL_rtf.
   
   == To be removed
   
  
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_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
  
  