There were lots of places in the code that called Sys_GrabInput(),
some of them each frame.
Most of this is unified in events.cpp now, in handleMouseGrab() which
is called once per frame by Sys_GenerateEvents() - this makes reasoning
about when the mouse is grabbed and when not a lot easier.
Sys_GrabInput(false) still is called in a few places, before operations
that tend to take long (like loading a map or vid_restart), but
(hopefully) not regularly anymore.
The other big change is that the game now uses SDLs absolute mouse mode
for fullscreen menus (except the PDA which is an ugly hack), so the
ingame cursor is at the same position as the system cursor, which
especially helps when debugging with `in_nograb 1` and should also help
if someone wants to integrate an additional GUI toolkit like Dear ImGui.
Minimum required Windows version is XP again (instead of Win10).
Win_GetWindowScalingFactor() tries to use two dynamically loaded functions
from newer windows versions (8.1+, Win10 1607+) and has a fallback for
older versions that also seems to work (at least if all displays have
the same DPI).
Moved the function to win_main.cpp so the dynamically loaded functions
can be loaded at startup; so edit_gui_common.cpp could be removed again.
Editor also seems to start, didn't test much further.
Only tested 32bit Windows, I fear the editor code isn't 64bit clean..
I hope I haven't broken anything elsewhere..
