Speed up PairPixelSampler

[nepfaff]

This speeds up the extremely slow PairPixelSampler. The main issue seems to be with mask erosion being slow. I managed to speed it up a bit but it still isn't great. I also added the functionality from #2585 to PairPixelSampler.

Before any of the changes:

With only erosion:

With both erosion and rejection sampling:

This is an initial step towards #3446

[nepfaff]

Not sure about the test failure:

ERROR: failed to authorize: failed to fetch oauth token: unexpected status from GET request to [https://auth.docker.io/token?](https://auth.docker.io/token?scope=repository%3Adocker%2Fdockerfile%3Apull&service=registry.docker.io:)

[akristoffersen]

Sweet, this is so much cleaner

[akristoffersen]

This looks great to me, thanks!

Is there a reason we aren't also using the rejection sampling for the PatchPixelSampler? Maybe not in this PR, but now the rejection sampling is its own separate fn it seems like it would be a drop-in replacement.

[nepfaff]

Is there a reason we aren't also using the rejection sampling for the PatchPixelSampler? Maybe not in this PR, but now the rejection sampling is its own separate fn it seems like it would be a drop-in replacement.

There is no reason, and it is a drop-in replacement. However, I agree that a separate PR seems better here. I opened one here (rebased this branch and started the new PR from it).