Weird stuff happened when dragging a GDI window between monitors
with different zoom (DPI) settings.  The following changes were
all done to clean up this stuff:

Window position is signed
Do some type casting for warnings
Fix return times of GetDpiFor*() pointers
Fix fallback to GetDpiForSystem() (could never be used)
Handle WM_DPICHANGED correctly
Move some copy/pasted code into a function
Fix return value for WM_GETDPISCALEDSIZE
Fix return value for WM_USER_SETCURSOR