Draft: Support for media in presentation-mode and seamless video playback

Should fix issue #869. Videos are now playable in the presentation mode.

Update: Media controls still do not work in presentation mode - someone has an idea why?

Additionally, the video playback becomes seamless both in the standard and presentation view. To get an impression, the following pictures are provided:

Previous state: Tiny black borders (about 1pixel left/right) are visible due to rounding errors:

In case aspect ratio of the player box does not match properly with the video, it appears as follows:

After the current commit, truly seamless appearance in both cases, border area is now "transparent":