Allow default text track at time 0 to be displayed

Created by: jeffcunat

For example with https://livesim.dashif.org/dash/vod/testpic_2s/xml_subs.mpd sample stream, there are captions at timestamp 0. There are not displayed because the video width and height is 0 before loadedmetadata fires. This PR avoid errors in imsc library when caption container has a size of 0. It also waits for loadedmetadata event to call checkVideoSize in order to calculate the caption container size at the right time to be able to display the first caption at time 0 (and not to wait next 500ms interval of the checkVideoSize call)