Allocate video surface object statically as a global

[smcv]

In #305, we have trouble with libsdl-perl making assumptions about the video surface never being freed or reallocated, and it being a no-op to "free" the video surface.

There is nothing that says a SDL 1.2 surface must always wrap the same SDL 2 surface, so we can maybe accommodate those assumptions by having the video surface be a special-cased global pointer that is never freed, and only reallocate its contents?

This seems to work with libsdl-perl's core_video.t test, but I haven't tested it extensively; consider it a proof-of-concept for now.

• Factor out Surface20to12InPlace

  This is a step towards making it possible to use the same SDL12_Surface
  for the video surface at all times, and only reallocating its underlying
  SDL 2 SDL_Surface.

• Factor out FreeSurfaceContents, and handle surface20 more defensively

  This is a step towards making it possible to use the same SDL12_Surface
  for the video surface at all times, and only reallocating its underlying
  SDL 2 SDL_Surface.

• Factor out enough of SDL_CreateRGBSurface to create surfaces in-place

  The part before we create the SDL 1.2 surface (creating the SDL 2.0
  surface) becomes CreateRGBSurface(), and the part after becomes
  Surface12SetMasks().

• Allocate video surface object statically as a global

  The SDL Perl bindings incorrectly call SDL_FreeSurface() on the result
  of functions that return a "borrowed" pointer to the video surface,
  namely SDL_SetVideoMode() and SDL_GetVideoSurface().
  (See Tries to free previous results of SDL_SetVideoMode(), leading to use-after-free crash PerlGameDev/SDL#305)

  When we would previously have allocated or freed the video surface
  wrapper object, instead allocate or free its contents in-place.

  When checking whether the video surface exists, because we never destroy
  it, we must now also check whether its underlying SDL2 video surface
  exists.

  Resolves: makes SDL_perl regress when it (incorrectly) frees a former video surface #305

[smcv]

@slouken, @icculus: I think this is ready for at least a preliminary "is this a good direction?" review, but it isn't sufficiently tested to land it yet.

[slouken]

This seems like a good direction to me.

[smcv]

Rebased on main and fixed a missing NULL check. Briefly tested with:

• libsdl-perl: frozen-bubble, pangzero
• non-Perl: amoebax, armagetronad, enemylines7, powermanga

and seems to work as intended.

[smcv]

I also contributed a fix for the incorrect free (PerlGameDev/SDL#306) which was applied to Debian's libsdl-perl package, but I haven't seen any sign of it being reviewed upstream. I also accidentally found PerlGameDev/SDL#307 while testing that change, and it hasn't had any response either, so PerlGameDev/SDL might be relatively dead.

[slouken]

Okay, well let's merge this then.

Thanks!