the Xcode project needs updating

[sezero]

Xcode project needs updating for both SDL2 and SDL3 branches:

• avif: vendored libavif has been updated to 1.0.1: the project must
  be updated to reflect that.

• dav1d: vendored dav1d has been updated to 1.2.1: the project must be
  updated to reflect that.

Additional notes about existing Xcode dav1d project in SDL_image:

• STACK_ALIGNMENT is defined only for x86_64 config but not arm
  configs: Intentional?? (Same goes for the Android configs in
  vendored dav1d itself.)

• STACK_ALIGNMENT is defined as 32: is that deliberately picked?
  Because meson build systems defines it either as 4 or 16 unless
  user-specified. (Same goes for the Android configs in vendored
  dav1d itself.)

• There is no config.asm in none of the configs. (Same goes for
  the Android configs in vendored dav1d itself.)

• • Only for Android configs in vendored dav1d itself: PREFIX is
    defined in all configs: Is that accurate, or some copy/paste
    artifact from Xcode configs in SDL_image? (I guess the latter?)

A job for @slouken (can't do myself.)

[slouken]

I don't know the answers to these questions, but I verified that the avif loader works on macOS. I'll go ahead and leave this open for now, in case someone runs into problems with the current settings.