Convert Sk4fGradientInterval to (bias, factor) format

And rename some fileds:

 * p0, p1   -> t0, t1
 * fC0, fDc -> fCb, fCg

Change-Id: I94b520f83d8f21d62c9c24740b6d3e2dbc3a9ff7
Reviewed-on: https://skia-review.googlesource.com/11799
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed