Fixes the off-by-one bug in RegisterInGrid(), which caused dtfile's desktop icons...

Fixes the off-by-one bug in RegisterInGrid(), which caused dtfile's desktop icons on the right edge of the screen (if desktop width is not a multiple of icon with) to be registered on the next workspace or (in the case of the last workspace) beyond the desktop_grid array (possibly causing segfaults).

On small screens segfaults could be also triggered without any icons on dtfile
startup if dtwm panel (or part of it) was registered beyond the screen when
RegisterInGrid() was called by InitializeDesktopGrid().

The patch also makes grid registration work for large objects (larger than
2 cells in any direction, like dtwm panel or icon with long file name).
Previously only rectangle vertices were registered.