Fix cuepoint positioning before video loaded
Fix cuepoint positioning before video loaded

The `calcLeft` method was erroring because the video length is unknown
until the video loads. It now checks first.

A nice side effect of this is that the cuepoints can be placed
off-screen until the video loads, instead of sitting at the start of the
scrub bar and then jumping to their correct positions when Play is
pressed.

electblake/videogular-cuepoints,electblake/videogular-cuepoints,HarryCutts/videogular-cuepoints,soton-ecs-2014-gdp-12/videogular-cuepoints,centsent/videogular-cuepoints,soton-ecs-2014-gdp-12/videogular-cuepoints,HarryCutts/videogular-cuepoints,centsent/videogular-cuepoints