iOS: honor SDL_HINT_MAIN_CALLBACK_RATE via CADisplayLink

[voki-andreev-a]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

On iOS, main callbacks are driven by a CADisplayLink, so SDL_HINT_MAIN_CALLBACK_RATE was ignored (unlike the generic path, which paces with SDL_DelayPrecise).

This change registers an SDL_AddHintCallback for SDL_HINT_MAIN_CALLBACK_RATE and, when a positive rate is set, applies it to the display link:

• iOS 15+ / tvOS 15+: preferredFrameRateRange via CAFrameRateRangeMake((rate * 2) / 3, rate, rate)
• older OS versions: preferredFramesPerSecond

Default behavior is unchanged when the hint is unset or "0" (including the existing ProMotion high-refresh setup). The hint can still be changed at runtime.

Note: the system may still choose a nearby supported frame rate (e.g. divisors of 60/120 Hz), as with other CADisplayLink preferences.

Existing Issue(s)

Fixes #15277