Pointer is displaced in EX0/EY0 and EX1/EY2 screen modes

[zedtoo-de]

Describe the bug
In the 1280x480 screen mode (in fact all EX1/EY2 modes) the mouse pointer is vertically misplaced by 50%. Also, in mode 22 (in fact all EX0/EY0 modes) the mouse pointer only appears in the bottom left quadrant of the screen. This is a long-standing bug, and may be cross-platform.

To Reproduce

1. Open Display Manager and select the 1280x480 screen mode.
2. Click and drag on the desktop background. Observe that the "marquee" is drawn below the apparent pointer position.
3. Press F12 and enter: *wimpmode 22
4. Press Enter to return to the desktop, then click and drag on the desktop background. Note this time that the "marquee" is drawn above and to the right of the apparent pointer position.

System information:

• Host machine: MacBook Pro 13-inch (2017 version)
• rpcemu-macos release: 0.9.1-pp3 (recompiler or interpreter)
• macOS version: Mojave 10.14.5
• Emulated processor: StrongARM Risc PC
• RISC OS version: 3.7, 5.24 or 5.27(06-June-2019 build)

Additional context
RPCEmu 0.8.14-caliston-dev1 also suffered from this bug.

[zedtoo-de]

I think this one also needs a "Help wanted" tag, as it may be cross-platform.