- This allows looking up the display index for an arbitrary location rather than requiring an active window to do so.
- This change also reimplements the fallback display lookup that found the display with center closest to the window's center to instead find the display rect edge
closest to the window center (this was done in the almost identical display lookup used in SDL_windowsmodes.c, which now uses `SDL_GetPointDisplayIndex`). In
practice this should almost never be hit as it requires the window's center to not be enclosed by any display rect.
- SDL_JoystickGUID -> SDL_GUID (though we retain a type alias)
- Operations for GUID <-> String ops are now in
src/SDL_guid.c and include/SDL_guid.h
- The corresponding Joystick operations delegate to SDL_guid.c
- Added test/testguid.c
