Initial SDL3 support.

[AliceLR]

Latest tested SDL3 build: 3.1.6.

• Update SDL3.
• Fix regressions for SDL 2.
• Fix regressions for SDL 1.
• Redo SDL pkg-config directory determination.
• Audio cleanup.
• Resolve #error in render_sdl.c.
• Resolve text input enablement (platform_sdl.c) and active check (event_sdl.c) issues. Fixed via Enable SDL text events at window creation time. #502.
• Resolve SDL_EVENT_WINDOW_RESIZED hack. Should be resolved thanks to Split set_video_mode into create_window and resize_window. #494 and Merge/cleanup renderer viewport calculation code. #496, needs testing.
• Resolve SDL_CreateWindow hacks. Switched to SDL3-style SDL_CreateWindow completely since MZX creates centered windows anyway.
• Resolve DSO hack TODO? No longer using SDL_FUNCTION_POINTER_IS_VOID_POINTER, Move SDL_LoadObject et al. to platform_sdl.c to fix warnings. #500.
• Resolve system mouse TODO? Done as part of Split set_video_mode into create_window and resize_window. #494.
• Mono support in SDL3 audio driver.
• Cleanup: something better than SDL_RenderTexture_mzx? Switched to just SDL_RenderTexture.
• Cleanup: SDL_VERSION et al. handling probably not ideal. Made X11 clipboard handler SDL1/2 only; defined new functions to get the compiled and linked version in SDL3 format instead of trying to rename the SDL1/2 macros/functions.
• Cleanup: unnecessary comments in some places.
• Cleanup: is it possible to handle some of the 0/-1 to true/false return value changes without #if/#else on branches?
  • SDL_Init Not plausible to do better than current.
  • SDL_GL_LoadLibrary Rewrote wrapper to take advantage of the changed return value.
  • SDL_GL_MakeCurrent Non-necessary call, removed completely.
  • SDL_GetClipboardText Not plausible to do better.
• Cleanup: final pass checking for any missed 0/-1 to true/false return value changes. Most of these never bothered checking the return value in the first place and should be safe by that technicality.
• SDL_WINDOW_FULLSCREEN_DESKTOP should remain in the GL allowed flags for SDL2.
• Should SDL_free the return value of SDL_GetClipboardText.
• Changelog (prior to merge).

Testing:

• software doesn't work (no window). Not clear if this was an upstream bug or fixed in MZX.
• software doesn't work in fullscreen (previous frame displays, no updates). Not clear what fixed this.
• software crashes in Linux. Cascading failure: resolution_width/resolution_height are unsigned, so -1 -> UINT_MAX, so software's minimum clamps on them fail, so MZX automatically selects the smallest resolution greater than or equal to 320x240, which happens to be 320x240. sdl_create_window_soft doesn't enable blitting for inadequate window surface sizes, so software writes out of bounds to the surface. Fixed all of these issues in Misc. fullscreen tweaks/fixes. #499.
• softscale doesn't work (black screen) (Linux only?). Caused by sdl_render_rect() normalizing coordinates.
• sdlaccel doesn't work (black screen). Caused by sdl_render_rect() normalizing coordinates.
• OpenGL doesn't initialize. SDL3 changed more 0/-1 return values to true/false.
• MegaZeux's software mixer incorrectly assumes the caller wants to fill the entire buffer, causing playback bugs when the requested frames count isn't exactly equal to the buffer frames count.
• SDL's ALSA, Pulseaudio, PipeWire, and JACK audio drivers all ignore SDL_HINT_AUDIO_DEVICE_SAMPLE_FRAMES. actually no it just works now???