Fix NaNs at 0 in ix_and_ptr() when rounding down.

Makes the clamp exclusive on left side as well so result is always
greater than zero and then does the ulp subtraction if required.

Bug: skia:13752
Change-Id: I9165e6baf2e823a419dcc79ab03c7c174098c72e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/617196
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed