Account for cells spanning multiple grid cells better when autosizing.

Account for cells spanning multiple grid cells better when autosizing.

The total size of a multi-span cell was accounted for each row/column it
covered, resulting in too much space being allocated to them.

Only take into account the average size of each row/column computed by
dividing the total cell size by number of rows/columns it occupies to fix
this.

Closes #11498.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63001 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775