475: VTT reader does not display cues with negative line attribute values

[pcochenn-harmonicinc]

Integer line values can be positive or negative. When positive, lines are counted from the top of the screen, and when negative, from the bottom of the screen.

The code did not handle negative values correctly, as line positions ended up greater than 100(%), hence out of the screen.
The fix handles negative values properly and adds a test case for both positive/negative integers and percentage.

Note that line value 0 should probably be handled as a positive value, but I did not change the current logic to remain backward compatible.

Closes #475

[palemieux]

Note that line value 0 should probably be handled as a positive value, but I did not change the current logic to remain backward compatible.

It looks like this should also be fixed since the spec states the following. I would fix it here unless you think a separate issue is necessary.

Positive line numbers count from the start of the video viewport (the first line is numbered 0), negative line numbers from the end of the video viewport (the last line is numbered −1).

[palemieux]

As discussed at #476 (comment) , I suggest fixing the line: 0 bug here as well unless you think a separate issue should be opened.

[pcochenn-harmonicinc]

You are absolutely right, thank you. I fixed the line:0 issue and added a test case for this.