add a linear gradient to P3 gm

I've tweaked the indicators into overlays so there can be one for each
pixel sampled.  This lets me test both gradient corners.

I've dropped displaying the P3 value... not sure if it's as useful as
the other two, and I'm worried about information overload.  There's
already a lot to look at.

Change-Id: I57a41c3af0162e8132e50c47a0b967714a57e9e5
Reviewed-on: https://skia-review.googlesource.com/154220
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
1 file changed