This read-only property returns true if the video is currently stalled, i.e. it has stopped
playing due to a lack of data, or too much data, but hasn't yet reached the end of the video.
This is set if the Video DOM element emits any of the following events:
stalled
suspend
waiting
And is cleared if the Video DOM element emits the playing event, or handles
a requestVideoFrame call.
Listen for the Phaser Event VIDEO_STALLED to be notified and inspect the event
to see which DOM event caused it.
Note that being stalled isn't always a negative thing. A video can be stalled if it has downloaded enough data in to its buffer to not need to download any more until the current batch of frames have rendered.