Alt/Option accelerators do not reach clients on macOS

[vejeta]

On macOS, Alt+letter accelerators (Motif mnemonics, e.g. Alt+X) never
reach the client, so menu accelerators do not work in xwpe.

KMOD_ALT is present and convertModifierState already maps it to
Mod1Mask, but on macOS Option also emits a character via SDL_TEXTINPUT,
so the combination does not surface as a clean Mod1Mask KeyPress the Xt
or Motif accelerator path recognizes.

Could libx11-compat map Option to Alt the way XQuartz does with its
"Option keys send Alt_L/Alt_R" setting (see SDL issue #12007)? That
would make accelerators usable for any Motif or Xt client on macOS.

Env: macOS, sdl2-compat 2.32.70 on SDL3 3.4.12.

[vejeta]

@jserv root cause and fix path.

The library side is already correct: convertModifierState maps KMOD_ALT
to Mod1Mask, and shouldSuppressIMTextKeydown excludes KMOD_ALT from text
suppression (both src/events.c). The break is upstream: with IME active,
macOS uses Option for character composition, so SDL delivers a composed
SDL_TEXTINPUT instead of a clean KMOD_ALT KeyPress and the accelerator
path never sees Mod1Mask.

Fix: set the SDL "Option as Alt" hint before video init
(SDL_HINT_MAC_OPTION_AS_ALT on SDL3, SDL_HINT_MAC_OPT_AS_ALT on SDL2),
in the existing hint block in src/display.c. Tradeoff: it disables
Option-based composition (typing n, a, ...), so I'd lean to gating it
behind an env var defaulting to off.

Happy to send a PR with a regression test once you pick a direction.

Env: macOS, sdl2-compat 2.32.70 on SDL3 3.4.12.

[jserv]

Happy to send a PR with a regression test once you pick a direction.

Please go ahead with proper test cases.