Added background gradient for fast-scrolling (closes #26)

[wilsonpage]

Seems to add ~50-80ms to a refresh tick. This patch applies the gradient in phase2, so this won't block first paint or content being interactive.

[wilsonpage]

@etiennesegonzac r?

[etiennesegonzac]

<3 the choice of gradient :) Looks like we need to offset the background though for the "search field" use case. (otherwise it looks like the field is fast scrolling :))

[wilsonpage]

@etiennesegonzac ah I see :) It's not that easy/possible to offset a repeating gradient is it? The simplest fix would be to apply a background-color to the search component. Any other suggestions?

[etiennesegonzac]

I wonder if a subtle use of background-position and repeating-linear-gradient could help, but applying a background color to the search component sounds good too :)

[wilsonpage]

@etiennesegonzac fixed the offset issue by programmatically drawing the gradient. Tested too.

[wilsonpage]

Happy to merge if you are :green_heart:

[etiennesegonzac]

Ship it!

[wilsonpage]

:ship: ed

[etiennesegonzac]

:)