Two more issues for the tracker

My colleagues Yuri and Anton have begun prototyping parts of the
Presentation API in Chromium and raised the following issues which I don't
believe are covered in the current specification.   Listed here and filed
in GH.

(1) Are multiple simultaneous calls permitted to startSession()?  My
intuition is "no", but I would like to ask the group if there are any use
cases where this is necessary.  If the answer is "yes" it adds some
constraints and complexity to the user interface for screen selection in
the browser.
https://github.com/webscreens/presentation-api/issues/21

(2) Can the web developer make any assumptions about the dimensions of the
viewport for rendering the presentation?  For example must it be a minimum
size, or must it match the aspect ratio of the display that is actually
rendering the content?  I believe the aspect ratio constraint is the most
important, but I could also see that a minimum size would help Web authors
with layout and CSS.
https://github.com/webscreens/presentation-api/issues/22