encoding-profile: Add a way to specify a factory-name when specifying caps
In some cases you want to ensure that a specific element factory is used
while requiring some specific caps but this was not possible. You can
now do qtmux:video/x-prores,variant=standard|factory-name=avenc_prores_ks
to ensure that the avenc_prores_ks
factory is used to produce the
'standard' variant of prores video stream.