[4fGradient] Relax interval checks

[4fGradient] Relax interval checks

We're currently considering intervals open at one extremity, in order to
ensure that a given value is always contained in a single interval.

This creates problems with synthetic clamp intervals [1, +inf), for
t == +inf (no interval can contain it).

Treat intervals as closed at both extremities instead.  This introduces
some ambiguitiy for overlapping values (contained in both adjacent
intervals), but solves the more serious problem above.

BUG=skia:6213

Change-Id: I33064f762fa9c2b914615e27977115d6654b12f4
Reviewed-on: https://skia-review.googlesource.com/8270
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>