Don't scroll the grid too much to bring selection in view.

Don't scroll the grid too much to bring selection in view.

wxGrid scrolled completely to the right in row selection mode as it was always
trying to make the bottom right selection corner visible. This was due to
adjusting the selection block corner to cover the entire row in this mode (of
course, the same was true for the column selection mode too).

Don't do this any more as it's unnecessary, making the real selection block
corner visible is enough for the block selection mode and nothing else is
needed in row/column modes.

See #12638.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65991 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775