Silence GAYLE
Amiga: Support PAL/NTSC switch for machines equipped with an ECS Agnus
chip or better.

Core changes:
- Save screen pixelclock.
- Call reset_partial_updates() in finish_screen_updates() instead of
creating an extra timer for it. This prevents calling
reset_partial_updates() after finishing the screen, causes issues when
screens change their size to a size lower than the current screen.
- Optimization: Support calling vblank_begin() directly at the start of
a frame instead of using a timer.