one option for fixing bilerp asserts

One option for fixing bilerp asserts is just to commit to clamping its
output.  Will write up another option that I think mirrors the GPU
backend more closely as a patch on top of this CL.

Few minor things here too:
   - Use clamp() more now that we have it.
   - Update assert_true() calls to print out the interesting value
     instead of the condition.
   - Clamp alpha before rgb for clarity in the case we're clamping rgb
     to that clamped alpha.  Either order actually works, but this order
     is more obviously correct.

Change-Id: I8f05834fd862ba8bb45b961e01563af091232c95
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/262811
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed