fileView.setImageSessionVars(): Consider only frames of single channel in case of lightsheet data
Eschew session variable currentFrameURL, instead frameURL() function is used

The point is that we may want to compose our image from multiple frames
from different channels in case of lightsheet data.

This breaks the #frameLink functionality; this doesn't seem very useful
to me (actually, not useful at all IMO) so I didn't bother to keep that
working, we can fix it later if needed.
webgl frameURL(): Factor out Images.findOne(), handle a weird case when this returns null
Lightsheet: Render both channels of all frames in parallel (using R and G channels)
Lightsheet box: Switch to blue so that it doesn't interfere with multi-channel rendering