video_player: frame accurate seekTo on iOS

[cbenhagen]

Same behaviour as on android. A keyframe accurate seekTo method could be introduced for both platforms to enhance performance at the cost of accuracy.

[googlebot]

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[cbenhagen]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

[mravn-google]

Thanks for the contribution. Please add pubspec.yaml version bump and CHANGELOG.md entry.

[cbenhagen]

@mravn-google done ;)

[mravn-google]

This branch seems to need a rebase now. Sorry.

[cbenhagen]

No worries. Branch has been rebased.

[cbenhagen]

Do you prefer the commits to be squashed when we are at it?

[mravn-google]

Thanks. I'll squash them as I merge.

[mravn-google]

LGTM