[Bug] `multimonitor` with fullscreen and a value greater than the number of displays results in out-of-range resolution requests

Steps to reproduce

1. Connect two external displays to a Win11 system.
2. Build EE off master and launch with multimonitor=3 (n+1, where n is the number of connected displays) and fullscreen=1.

Expected behavior

Either:

• EE launches into full screen on each display, each display runs at its native resolution, and a third "display" is opened as a window.
• EE ignores fullscreen=1 and opens three windows.

Observed behavior

EE launches into full screen on each display, but each display cycles endlessly into attempting to use an out-of-range resolution.

EE logs:

[DEBUG   ]: SDL_GetDisplayBounds(display_nr, &rect) returned -1. display_nr: 2
[ERROR   ]: SDL error in Window::calculateWindowSize() at SDL_GetDisplayBounds(display_nr, &rect): displayIndex must be in the range 0 - 1
[WARNING ]: SDL_GetDisplayBounds() returned a rect with at least one dimension < 240: 0,0
[DEBUG   ]: Window dimensions before scaling loop: 1200,900
[WARNING ]: Window::calculateWindowSize() couldn't solve windowWidth and windowHeight in 16 attempts: 218,163
[DEBUG   ]: Window::calculateWindowSize() reported at least one window dimension of < 240: 218,163
Falling back to 640,480.

Workarounds

• Exit full screen (Alt+Enter) on the primary display after launch. The mode switching loop stops and EE can be exited.
• Set fullscreen=0 and use windowed mode. EE opens three windows.
• Ensure that the number of displays matches the value of multimonitor. Re: #2701 these displays should likely also have the same native display resolution.

Start at Window::calculateWindowSize() and the SDL_GetDisplayBounds() calls shown in the logs, reproducing with two displays, multimonitor=3, and fullscreen=1 on Windows 11. Trace how the out-of-range display index is handled. Done means the configuration no longer loops on invalid resolutions and follows one of the expected fullscreen or windowed behaviors.