Before exporting images, change the background to transparent

Also enables scaling exported graphics (x2 by default).
Unfortunately, gives a "flash" when the background changes, but
I'm not sure how (or if) to stop this (it indicates a snapshot).

Change-Id: Idcb72b1a395f779cab0d58b94d4f4b0a112d3ad0
Merge pull request #5 from ghutchis/ENH_export_transparent_backgrounds

Sorry I didn't circle back sooner, I will push a minor fix for the GLRenderer class shortly, the way we save the bitmap here causes initializeGL to be called, which invokes initialize in GLRenderer and that had a resetCamera call, causing the camera to reset every time and the exported bitmap. It removes one line of code, and resetCamera is called in other places when a new view/molecule is loaded.