Cache HDC used for painting for the entire duration of WM_PAINT processing.

Cache HDC used for painting for the entire duration of WM_PAINT processing.

This fixes a long standing problem with 2 wxPaintDC created one after another
(and not with nested lifetimes, which was handled by the caching mechanism
previously used) not working correctly. And as this was exactly what happened
when handling wxEVT_PAINT for wxScrolled, it also fixes drawing artefacts when
using scrolled windows.

Closes #14757.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72938 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775