Clean copy of ImageSequenceGrabber. [1546]

This is a temporary override.
Handle component groups in "movie" captures.

[1546]
Add "bgtransparent" parameter to captureImage.

Default value is False, which makes captureImage behave exactly the
same as it did before this commit.

If bgtransparent=True, the current implementation just ends up calling
“writeImage(filename, ‘backgroundtransparent’, quality)”…which means
that this doesn’t work so well for multiple displays.

[1843]