WIP: delete glSDL shim, replace with sdl_compat.h

Step 1+2 of the SDL 3 migration. The 2300-line graphics/glSDL.{c,h}
OpenGL-on-top-of-SDL-1.2 shim is obsolete under SDL 3 (SDL_Renderer
does what glSDL did natively), and its #define overrides of SDL
surface APIs collided directly with SDL 3's SDL_oldnames.h.

- Add graphics/sdl_compat.h: a one-line umbrella that just pulls in
  <SDL3/SDL.h>.
- Replace every #include "glSDL.h" with #include "sdl_compat.h" (15
  files across the tree).
- Delete graphics/glSDL.c, graphics/glSDL.h.
- Drop glSDL.c from graphics/CMakeLists.txt.
- Remove GFX_DRIVER_GLSDL from the gfx_drivers_t enum and from all
  switches/conditions in gfxengine.cpp.
- Drop the glSDL_VSync() call site (will be re-introduced as
  SDL_SetRenderVSync once gfxengine is ported to SDL_Renderer).
- Drop the two glSDL_Invalidate() calls in window.cpp (texture
  re-upload hints with no equivalent in a pure-surface compositor).
- options.cpp: remove the now-empty "OpenGL/glSDL" menu item; force
  videodriver to GFX_DRIVER_SDL2D.
- prefs.cpp: collapse legacy GLSDL setting onto SDL2D on load.

After this commit the macro-collision wall is gone (renamed_-token
errors drop from many to zero). The build still fails on the next
layer of work: SDL 1.2 surface APIs that no longer exist
(SDL_FreeSurface, SDL_SetAlpha, SDL_CreateRGBSurface, SDL_DisplayFormat,
SDL_PixelFormat struct member access, SDL audio callback API, etc.).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

sound/a_tools.h

@@ -22,7 +22,7 @@
 #ifndef	_A_TOOLS_H_
 #define	_A_TOOLS_H_
 
-#include "glSDL.h"
+#include "sdl_compat.h"
 #include <string.h>
 #include "kobolog.h"
 #include "a_types.h"

sound/a_types.h

@@ -22,7 +22,7 @@
 #ifndef _A_TYPES_H_
 #define _A_TYPES_H_
 
-#include "glSDL.h"
+#include "sdl_compat.h"
 
 /* Builtin patch drivers */
 typedef enum