Fix the checkbox cell size in generic wxDataViewToggleRenderer.

Fix the checkbox cell size in generic wxDataViewToggleRenderer.

r62940 fixed an appearance problem with the checkboxes in wxDataViewCtrl but
introduced another one: as the checkbox was now always drawn in the entire
cell rectangle, the cell alignment was not taken into account any more.

Fix this by only increasing the checkbox rectangle up to the required minimal
size but not any more.

Closes #14504.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72294 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775