[Shared] Add cl_mouseAspectScaling to correct UI mouse sensitivity for your resolution

[Razish]

With this enabled, the cursor moves about the same rate as my desktop cursor.
Without this, horizontal movement is too sensitive on e.g. 1920x1080 windowed.

It's optional behind a cvar because without a breaking API change[1] or a more complex solution[2], small (sub-pixel) movements are completely discarded by being truncated.
It may have also already been fixed by a mod[3], so we don't want to scale it twice.

[1] int -> float for UI_MOUSE_EVENT / CG_MOUSE_EVENT
[2] e.g. floating point accumulation buffer that emits movement events when it exceeds a whole number
[3] ja++ equivalent fix in ui/ui_main.cpp

[bartrpe]

So I had it tested across various things:

• two merged 1920x1080 screens (essentially 3840x1080... I should one day get to try to list all the various bugs that happen at this ultra wide level)
• one 1920x1080
• tested every resolution set that's preset at r_mode options
• tested on 3840 x 2160 TV
• tested on old 1280x1024 screen

did that with various mouses too because why not

no bugs found