macOS: fix crash on document open with sdl2-compat

[richstokes]

Follow-up to #2492. When Flycast is built against Homebrew's sdl2, which is now sdl2-compat on top of SDL3, launching with a game on the command line segfaults during window creation:

SDL_strdup_REAL
Event3to2                      (libSDL2-2.0.0.dylib, sdl2-compat)
EventFilterWrapper3to2
SDL_FilterEvents_REAL
...
SDL_CreateWindow_REAL
sdl_recreate_window(unsigned int)   sdl.cpp

AppKit also delivers the command-line game through application:openFile:, which pushes an SDL_DROPFILE event. sdl2-compat's Event2to3() has no case for drop events, so the SDL2 drop.file pointer doesn't end up in SDL3's drop.data. When the queued event is later converted back, Event3to2() calls SDL_strdup(event3->drop.data) with NULL. Official builds bundle real SDL2, so they aren't affected.

This keeps the design from #2492 (the document waits in SDL's event queue until the main loop is running, without setting commandLineStart) but carries the path in an event type from SDL_RegisterEvents(). User events keep the same layout in both SDL versions, so data1 survives the round trip. The queueing moves into sdl_queue_open_file() in core/sdl/sdl.cpp next to the handler that frees the string.

Tested on Apple Silicon, macOS 27, native CMake build of this branch:

SDL	game on command line	open -a Flycast.app game.elf
Homebrew sdl2-compat 2.32.72 / SDL 3.4.16	boots (crashed before)	boots
bundled SDL2	boots	boots