When setting an encoding profile from a file we need to make sure
several restrictions are handled:

 - We need to make sure the resulting restriction caps are
   compatible with the encoder that is going to be used by encodebin

 - We should ensure that the profile restriction caps are fully taken
   into account (if those restriction are not compatible with the encoder
   we can't do much)

 - We need to try as much as possible to use user previously set formats

 - We need to ensure fields that are mandatory for us are set in a way
   that is compatible with the encoder

This introduces a utility function (for better testability) that allows
this kind of caps fixation and some unit tests for this function.

Reviewed-by: Alex Băluț <&lt;alexandru.balut@gmail.com&gt;>
Differential Revision: https://phabricator.freedesktop.org/D1807