[Bug] `multimonitor=1` at fullscreen using multiple resolutions results in perpetual mode switching loop

[oznogon]

Steps to reproduce

1. Connect two external displays to a Win11 system, each with a different native resolution.
2. Build EE off master and launch with multimonitor=1 and fullscreen=1.

This also affects multimonitor=2 with two displays. A separate but likely related issue affects multimonitor=n+1 (#2702).

Expected behavior

EE launches into full screen on each display, and each display runs at its native resolution.

Observed behavior

EE launches into full screen on each display, but both displays perpetually switch fullscreen resolutions, resulting in an endless loop of mode switching (blackout, input cycle, resolution change, blackout, etc.).

Workarounds

• Exit full screen (Alt+Enter) on the primary display after launch. Full screen remains active on the second display, and the mode switching loop stops.
• Set fullscreen=0 and use windowed mode on both displays.
• Ensure that both montiors use the same native display resolution. This was reproducible only with monitors of different resolutions (4k and 1080p in testing); monitors of the same resolution (1080p both in testing) work fine.

[daid]

I wonder if this is a bug in SDL2 or EE... as I don't think we touch the window size after creation except when changing to fullscreen or not.