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_mutex.h'
  * 'SDL_quit.h' (only necessary when using `SDL_Main`?)
  * 'SDL_thread.h'
  
  == Nothing to implement
  
  
negative.code: ** `SDL_GetRenderTarget` (renderer)
  ** `SDL_RenderIsClipEnabled` (renderer)
  ** `SDL_RenderReadPixels` (renderer)
  * '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
  ** 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_GetWindowSurface` (window)
  ** `SDL_UpdateWindowSurface` (window)
  ** `SDL_UpdateWindowSurfaceRects` (window)
  
  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 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.
  
  == OpenGL and Vulkan
  
  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.
  
  The following functions are related to OpenGL and Vulkan and still
  need to be implemented:
  
  * 'SDL_render.h':
  ** `SDL_GL_BindTexture`
  ** `SDL_GL_UnbindTexture`
  * 'SDL_video.h':
  ** `SDL_GL_LoadLibrary` (unclear if we need it)
  ** `SDL_GL_GetProcAddress` (unclear if we need it)
  ** `SDL_GL_UnloadLibrary` (unclear if we need it)
  