Fixed cursor centering for odd window sizes.
Fixed reporting of negative window positions.

MSDN recommends LOWORD and HIWORD for WM_MOVE, but these do not
handle negative window positions correctly.

Fixes #172.
Added note.
Added credit.
Avoid X11 video mode setting if modes match.

This matches the behavior of the Windows port.
Fixed warnings from Clang static analysis.
Merge remote-tracking branch 'upstream/master'