Fix for image editor plugin not scaling loaded bitmaps

On Windows7 x64 with Aero enabled, Python 2.7 x64 and
wx-2.8-msw-unicode x64, bitmaps loaded into the image
editor (.png, .bmp, etc) don't appear scaled as per the
scale slider. Instead, they appear in the top left corner
unscaled.

Bitmaps created from scratch do scale properly.

This check-in adds a workaround that makes loaded bitmaps
mimic the created bitmap code (create a new HDC & bitmap, blit
from the loaded bitmap to the HDC, rather than create
a new HDC, select the loaded bitmap into the HDC)