[4fGradient] Relax interval checks for SkGradientShaderBase also

Similar to https://skia-review.googlesource.com/8270, treat intervals
as closed at both extremities in the 4f gradient fallback impl also.

BUG=skia:6212

Change-Id: I7f164868202ae6a0f76cbcdbcbf8e62db12a1bd4
Reviewed-on: https://skia-review.googlesource.com/8277
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
4 files changed