Use 'webvtt_parse_cuesetting' for 'size' cuesettings

I've also updated the cssize_unittest test set by using the helper
function 'expectDefaultSizeSetting()' rather than explicitly asserting
individual vertical settings. I've also changed some assertions that
are not really fatal to be expectations instead, such that they will
still cause a failure, but also provide additional information with
subsequent failing tests.
Merge pull request #327 from caitp/size-setting

Use 'webvtt_parse_cuesetting' for 'size' cuesettings