Simplify wxGridCellAutoWrapStringRenderer::GetBestSize().

Simplify wxGridCellAutoWrapStringRenderer::GetBestSize().

The code there was pretty wild, making clearly wrong assumptions (column size
doesn't, and AFAICS never did, include 20 pixel margin) and also was clearly
uncertain about what it was doing by trying to limit the number of iterations
to some arbitrary cutoff when it is pretty clear that the loop increasing the
width and decreasing the height on each iteration will reach the condition of
"width >= height*1.68" sooner or later.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74751 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775