This replaces the Windows-only code with code that should work on
all the platforms. Tested on Linux/Wayland.

The “Scaling” option is now just treated like a normal UI zoom
control for users who want a larger UI. This means “Native” scaling
option is removed now since that does not mean anything.

Rendering at a non-native resolution and then scaling the bitmap
looks bad. The scaling seems to try to just get it to end up the
same size as the welcome screen, so just use the same size for
both.

This change also future-proofs the scaling event so it can be fired
when users change the scaling mode in the settings, since both user
scaling and window content scaling changes require
re-rasterisation.