Fix menu mouse position on high pixel density displays

[AllMelody]

SDL_GetMouseState returns points, M_PixelToMenuCanvasCoord expects pixels. IN_GetMousePos scales between the two; the console already uses it.

No change when pixel density is 1. Not tested on Windows or Linux. Fixes #1011 locally for me.

[vsonnier]

In original report:

To Reproduce
Open vkQuake 1.36.0 in windowed mode on a high DPI screen.

To confirm, was it just windowed that was wrong, or also fullscreen ?

I"ll test for Windows, thank you for the fix. @Perlence do you confirm the bug, and the fix on your side ?

Hum. the #ifdef USE_SDL3 guards are gone there.
So, was it for SDL2 or SDL3 ?. I don't think we want to fix hiDPI issues on anything except the newer platform SDL3, to ease maintainance burden and prevent regressions on the different platforms.
What is your opinion @Novum ?

[Perlence]

Can confirm, in 1.36.0, mouse movement doesn't select the menu items in both fullscreen and windowed modes. However, the console text selection works:

Screen.Recording.2026-09-01.at.22.29.04_edit.mov

The fix works:

Screen.Recording.2026-09-01.at.22.33.25_edit.mov

[vsonnier]

OK @AllMelody Now that I really looked at the code :) your patch makes perfect sense, we don't need the SDL3 guards at that level. Works for me windowed/fullscreen on Windows.

I don't see what could break here. Thanks for your contribution ! and thanks @Perlence for the confirmation !