[PATCH 1/3] comctl32/treeview: Erase background on WM_PAINT if BeginPaint() tells us to do it

[Nikolay Sivov]

This fixes at least two currently open bugs (one for Canon DPP).
Problem is that we shouldn't ignore fErase flag returned by BeginPaint()
in a PAINTSTRUCT. If application subclassed ListView and doesn't pass (blocks)
WM_ERASEBKGND to our original control procedure we should still be able to
erase background at WM_PAINT stage.

Changelog:
    - erase background on WM_PAINT if BeginPaint() tells us to do it