Mask expansion 'x' - missing description

[mgrudzinska]

'x' is present in the sample: https://lottie.github.io/lottie-spec/1.0/specs/helpers/#mask
but there is no description of it.

After some tests in AE it seems that the line join type "round" has to be force. Also it seems that it is not affected by the path direction. Would be nice to have this info in specs.

[mbasaglia]

Mask expansion is not in the specs because it isn't very well supported by players, it's described very briefly on lottie docs though: https://lottiefiles.github.io/lottie-docs/layers/#masks

[mgrudzinska]

Out of more than 10k tested files with masksProperties, less than 1% had a non-zero mask expansion value.

[mgrudzinska]

#123