AlphaPicker with vertical direction is upside down
this may be subjective, but the vertical alpha picker has the 100% alpha point at the bottom, which is counterintuitive from a UI perspective. in the docs i don't see an option to change this. is there a workaround?

a secondary issue is the scroll handle being off center from the track.

<AlphaPicker direction="vertical" />

here's a screenshot to illustrate:

<img width="72" alt="screen shot 2019-01-21 at 11 02 53 am" src="https://user-images.githubusercontent.com/1611256/51486212-135d4b00-1d6e-11e9-800d-75e64d0b5480.png">
