[Media] Add Media Fragments URI to control, distribution, and rendering pages

[tidoust]

As suggested in:
https://lists.w3.org/Archives/Public/public-web-and-tv/2017Oct/0004.html

@rtroncy, could you take a look and let me know if the text works for you? I hesitated before I included the spec in control, distribution and rendering, but I agree with you that it indeed addresses these 3 aspects.

[rtroncy]

Yep, thanks, I like the 3 paragraphs. Note that media fragments URI can be made both for the temporal dimension and the spatial one (cropping), and that for the temporal position, it is possible to specify the starting time and the ending time (to not view the video until the end) ... but temporal + start is 95% of the usage so you're good :-)

[tidoust]

OK, this sort of document will probably never be good at capturing nuances so 95% is not so bad ;)

I had not mentioned the spatial dimension because my understanding is that main browsers do not support it so far, but it may be worth emphasizing cropping for other use cases. Suggested text would be of course absolutely welcome... I'm merging this for now, thanks for the review, @rtroncy!