Update the column width immediately, as it's being dragged, instead of
drawing a temporary line showing the new column boundary using wxINVERT.
This results in better user experience, as it the effect of changing the
column width can be immediately seen (especially important for non-left
aligned columns or columns using ellipsizition) and, equally if not more
importantly, fixes wxGrid drag-resize not showing any visible UI at all
with wxGTK3 and wxOSX where wxINVERT is not implemented.
8b2237cd2d