Add comment about keeping libxcb-cursor copy of code in sync

_XcursorThemeInherits, XcursorWhite, & XcursorSep are copied in libxcb-cursor/cursor/load_cursor.c and should be kept in sync with changes to the libXcursor originals of those.

Hoping to avoid the need for further resyncs like libxcb-cursor!5 (merged) when they've drifted apart.