Reduce the number of realloc() calls in wxHtmlTableCell.

Don't call realloc() after adding each row, this is horribly inefficient, so
preallocate more memory to avoid this.

Closes #15931.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75742 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775