SDL Now Supports DOS

Seeking breaks otherwise. We might be able to just fflush() before or seeking
instead?

Turns out DosBox-X was having trouble with the Sound Blaster or something;
standard DosBox works correctly directly from the interrupt handler, and
without doubling the buffer size.

This is MUCH faster than just leaving buffering disabled, and also works
around getting bogus reads after an fseek. SDL_LoadWAV on test/sample.wav
no longer takes several seconds to finish, and comes up with the correct
data.

I wonder if we're triggering this in LoadWAV because we're malloc'ing data
between seeks/reads, and it's causing the djgpp transfer buffer to change. Or
maybe the Fat DS trick is confusing it? I don't know, I haven't had time to
debug it, it might just be a legit libc bug in djgpp too, for all I know.

This uses an old trick we used in SDL 1.2 for MacOS Classic, which did its
audio callback in a hardware interrupt. If the audio is locked when the
interrupt fires, make a note of it and return immediately. When the lock is
released, if the interrupt has been fired, run the audio device iteration
right then.

Since there isn't a big device lock in SDL3 (available to the app, at least),
this keeps a counter of when any SDL_AudioStream is locked, which is probably
good enough.

This uses VESA interfaces to manage the display and works with the software
renderer.

Events aren't hooked up yet, so prepare to close DosBox on each run.  :)

This gets most of the rendering examples, which use SDL_GetBasePath() to
find textures to load, working.

Of course Quake 1 solved this better, haha. It's smart: less memory, dirt
simple, and you don't even have to worry about synchronizing with the
interrupt handler, because it's safe for both sides no matter when an
interrupt fires.

[sdl-ci-filter djgpp]
[sdl-ci-artifacts]

- SDL_runapp.c: Add SDL_PLATFORM_DOS to the exclusion list so the
  generic
  SDL_RunApp() is disabled when the DOS-specific one is compiled.
- SDL.c: Exclude SDL_Gtk_Quit() on DOS. DJGPP defines __unix__ which
  sets
  SDL_PLATFORM_UNIX, but DOS has no GTK/display server. The GTK source
  is not compiled (CMake UNIX is false for DOS) so this was a link
  error.
- sdlplatform.cmake: Add DOS case to SDL_DetectCMakePlatform so the
  platform is properly detected from CMAKE_SYSTEM_NAME=DOS.
- i586-pc-msdosdjgpp.cmake: Add i386-pc-msdosdjgpp-gcc as a fallback
  compiler name, since some DJGPP toolchain builds use the i386 prefix.

- Implement double-buffered page-flipping for VBE modes with >1 image
  page
- Save and restore full VBE state on video init/quit for clean mode
  switching
- Improve DOS keyboard handling: support extended scancodes and Pause
  key
- Lock ISR code/data to prevent page faults during interrupts
- Always vsync when blitting in single-buffered modes to reduce tearing

Move audio mixing out of IRQ handler to main loop for improved
stability and to avoid reentrancy issues. Add SDL_DOS_PumpAudio
function, update DMA buffer handling, and adjust sample rate to 22050
Hz.
Silence stale DMA buffer halves to prevent stutter during load.

Detect SB version and select 8-bit mono or 16-bit stereo mode.
Handle DMA and DSP setup for both SB16 and pre-SB16 hardware.
Add FORCE_SB_8BIT option for testing in DOSBox.

- Poll Sound Blaster DSP status instead of fixed delay after speaker-on
- Clarify DPMI conventional memory is always locked; update comments
- Document and justify DMA memory allocation strategy
- Free IRET wrapper after restoring interrupt vector to avoid leaks
- Throttle joystick axis polling to ~60 Hz to reduce BIOS timing loop
  cost
- Always poll joystick buttons directly for responsiveness

Implement banked framebuffer access for VBE 1.2+ modes without LFB.
Detect and initialize banked modes, copy framebuffer data using bank
switching, and blank the framebuffer on mode set. Page-flipping is
disabled in banked mode.