comctl32/treeview: Erase background when painting over supplied hdc

[testbot at testbot.winehq.org]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=5489

Your paranoid android.


=== W98SE (32 bit treeview) ===
treeview.c:1293: Test failed: got 0x0