Added a hint SDL_HINT_MOUSE_RELATIVE_SYSTEM_SCALE

[slouken]

Added a hint SDL_HINT_MOUSE_RELATIVE_SYSTEM_SCALE to control whether to use system mouse acceleration on raw relative motion.

Description

This is currently a draft, partially implemented on Windows. The linear scaling is correct, but the "Enhanced pointer precision" mode is not quite correct. Feedback welcome!

testsprite2.c has been modified to show a green cursor where the real system cursor is, and a blue cursor where SDL thinks it is, so you can easily see where they differ. SDL_windowswindow.c has a change for testing to clip the cursor to the window in relative mode and SDL_windowsevents.c has a change for testing to send the real mouse cursor position to testsprite.

Existing Issue(s)

Fixes https://github.com/libsdl-org/SDL/issues/5992