Fix issue #464, repeated/skipped frames in ImageSequenceClip

[neitzal]

This PR fixes the occasional repeating and skipping of frames mentioned in issue #464 due to floating point inaccuracies in images_starts.

[coveralls]

Coverage decreased (-0.02%) to 47.401% when pulling b7de3cc on neitzal:issue_464 into 0b40803 on Zulko:master.

[ghost]

@neitzal, would it be possible for you to add a test for this that would fail before your fix, but succeeds after your fix? You can post it here or you can add it to tests/test_issues if you want. Is that okay?

[coveralls]

Coverage increased (+0.2%) to 47.651% when pulling 7951e07 on neitzal:issue_464 into 0b40803 on Zulko:master.

[neitzal]

@Earney Sure, I added a test for one failure case that now succeeds. The test is not exhaustive though, in the sense that it does not verify that no fps-number (in a reasonable range) ever produces a frame mismatch. I guess that would be a little too expensive to run every time.

[ghost]

Awesome.. Thanks!

[ghost]

@Zulko do you think this would be okay to merge into the repo? I'm not an expert on ImageSequenceClip so I'm a bit hesitant.

[ghost]

any objections to this PR?

[Zulko]

LGTM