Using color rather than opacity to mask

Using color rather than opacity works better with linear gradient layers masks and lets you use jpg assets rather than png/gif ones for masks.

Currently the Rdio embed clients are broken*-- am not sure how things drifted into that state because it did use to work.  After writing tests for the Linear Gradient I think I figured out how to fix the Rdio embed without changing the code**, but I'm going ahead and posting this because I think it's a better solution.  

- * I know this repo is public, but for internal use see T23027

- ** The '_' layer is black & solid.  We'd have to make a new opacity 0 layer to overlay the linear gradient on but that seems overly complex.