fix: consistent fade-in and fade-out on start end exit of nextui and apps

[nborodikhin]

Description

SDL apps always show a black frame on start, that's unavoidable - SDL_CreateWindow always fills screen buffer with black pixels. On light themes, this leads to a jarring experience with multiple flashes of black.

Observations:

• black screen on start unavoidable, we should embrace the fact, and use opening/closing animations to avoid flashes
• opening NextUI has a fade-in animation (it causes background flickering on light background due to bad orchestration though)
• starting a game in NextUI has an opening animation (defunct, but the code is still there)
• opening emulator has 8 frames of opening fade-in animation
• exiting emulator has a fade-out animation

This PR extends the ideas and observations and adds (and re-adds) a fade-in animation on start and fade-out animation on exit, both for NextUI and built-in apps. It complements #854 which improves light theme support on built-in apps.

Change

• CFG_getAppStartExitAnimationDuration():
  • consistent and stable way to get the fade in/out duration
  • use standard durations 20/150/250
• GFX_init():
  • keeps the screen black, sets the background color without a present, so a subsequent fade-in animation could start from black
    -GFX_animateBlack()
  • runs and animation mixing captured screenshot and black frame
  • a fade-out also leaves the screen texture black, thus the exit clear in GFX_quit() stays black
• nextui.c:
  • fade the first frame in from black after the pill and the art are drawn
    • this solves an issue with only parts of the screen being drawn today
  • do not draw more ui frames once it decided to start a game or pak
    • this solves an issue with only parts of the ui being visible during animation
  • fade out to black before exit (note that exit always starts a app)
• battery.c, clock.c, gametime.c, ledcontrol.c, minput.c, settings.cpp:
  • fade in on the first frame, fade out on exit

Testing

• tested on tg5040
• Default and Catppuccin Latte themes
• open Settings from the game list and quick menu: the quick menu fades out, not the game list
• cold boot: the launcher fades in from black