Detect clock sync errors in MPDs

[sandersaares]

There is some sort of dynamic timing conformance validation supported, it seems, but it is rather incomplete.

Example: http://vm1.dashif.org/DynamicServiceValidator/?mpdurl=https://dmpe12-4dash.akamaized.net/dash/live/594304/DPME12-4/index.mpd

This gives an error because the validator apparently does not support $time$. The validator should be enhanced to support this.

The referenced MPD above is believed to have incorrect timing (content starts in the future). Having the validator specifically confirm this would help improve interoperability in the packager ecosystem, as without it there is very little possibility to make it clear to packgers that they produce invalid content. This content came up in discussion here: https://github.com/google/shaka-player/issues/1232#issuecomment-362349850

I will collect some more cases here:

• https://github.com/Dash-Industry-Forum/dash.js/issues/2470
• https://github.com/google/shaka-player/issues/1385

[dsilhavy]

@Phencys Related to the discussion on the support template mechanisms