COMMENT: Added comments clarifying the previous WAR

It looks like the WAR  in commit 44477d33ff641663b0e6b0716c45ac2cb5fab349
is wxWidgets issue 3494 http://trac.wxwidgets.org/ticket/3494

The problem is when a DIB is selected onto a DC and a
StretchBlt sources from that DC, the units used need to be
device units but wx uses logical units.

This matches the WAR observations, where using a scratch
BMP instead of the incoming image (which is likely a DIB)
fixes the issue.
ENHANCEMENT: Added flicker free redraw to ImageEditor.plug-in.py

The image editor was flickering badly on Win7x64, Aero disabled
so I added flicker free redraws by:
1. Ignoring the EraseBackground event for the edit window
2. Buffering the draw to an AutoBufferedPaintDC (this is a nop on
   OSs that already have doublebuffered drawing)
3. Clearing the background in OnEditWindowPaint

Other option would be to only clear to background the parts of the
window that won't be covered by the bitmap, but that doesn't prevent
the grid xor drawing on top from causing flicker.

Tests:
Loaded several png and jpg files, verified there's no flicker and
that the background is cleared properly with and without scrollbars.