Fix wxGrid editors background painting.

Fix wxGrid editors background painting.

There were two fundamental problems: first, we painted on a separately created
wxClientDC instead of using the wxPaintDC already available in wxGrid. Second,
we invalidated the control while painting, resulting in endless repainting, at
least under wxGTK.

Fix the first problem by passing wxDC to wxGridCellEditor::PaintBackground()
and the second one by not refreshing the control from there as it just seems
unnecessary.

Also pass the attribute by reference for consistency with
wxGridCellRenderer::Draw() and because this pointer can never be NULL.

Closes #2628.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72697 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775