RaP: Adjust the 'convert' command to make opaque images

This forces a background colour of white on the generated png images for
slides in the slides and presentation recording formats. Fixes Issue
1588.
Merge pull request #213 from kepstin/opaque-recording-slides

RaP: Adjust the 'convert' command to make opaque images
Merge branch 'master' of git://github.com/bigbluebutton/bigbluebutton