Switching between RTG modes and native modes doesn't update screen

[spectrumero]

Describe the bug

Built from source, commit e535218c

When the emulated Amiga switches between a native mode (e.g. normal NTSC or PAL screen mode) and an RTG mode, or from an RTG mode to a native mode, the emulator continues to show the old screen mode (and sometimes the old screen contents) until you press F12 to bring up the GUI and dismiss it again.

To Reproduce
Steps to reproduce the behavior:

1. Load amiberry-lite with a configuration that switches to an RTG mode, e.g.:
   amiberry-lite --conf fancy_wb_conf.uae
2. During the boot process, Workbench switches to the RTG mode but the screen doesn't update (you just have a grey screen in the emulator).
3. Press F12 to bring up the GUI and dismiss it again. The emulator will now show the RTG mode screen.

Others:

1. From Workbench, run a program that switches from RTG to a native mode.
   Note that the RTG workbench screen still shows, but frozen.
2. Press F12 to bring up the GUI and dismiss it again. The emulator will now show the native mode screen.

Expected behavior
The emulator should switch from native to RTG mode and vice versa. Last version I used where this worked correctly was amiberry-lite 5.8.11

Screenshots
Video of the bug happening: https://spectrum.alioth.net/amiberry-lite-rtg-bug/amiberry-lite-modeswitch-bug.mkv
Video of the behaviour of 5.8.11: https://spectrum.alioth.net/amiberry-lite-rtg-bug/amiberry-lite-5.8.11-no-modeswitch-bug.mkv
Link to the screen test program with source: https://spectrum.alioth.net/amiberry-lite-rtg-bug/screentest.lha
(a precompiled version of screen is in the archive)

The RTG mode it boots into is 1280x720, but the RTG mode isn't important, it's the switch between RTG and native or native and RTG.

Desktop (please complete the following information):

• OS: Debian 12 (x86_64), and Raspberry Pi Raspbian (aarch64)
• Version: 5.9.0