fix multiple issues with non-window scrolling context

[jedwards1211]

This fixes the following issues when scrolling with a context besides the window:

• it was using 0 for the start position instead of the context's scrollTop
• it wasn't determining the end position correctly (should be el.getBoundingClientRect().top - context.getBoundingClientRect().top + context.scrollTop)
• smooth scrolling to the end wouldn't ease out in some cases. Solution: clamp the end position to context.scrollHeight - context.offsetHeight

Note that this still doesn't work if there are nested scrolling elements between the context and the target element.

Also, this includes my other PR by accident. Let me know if you want to remove that PR from this.

[alicelieutier]

Hey, I'd like to remove the other PR from this. Other than this, I'm happy to merge this patch.
I'll comment on the other one individually.

Sorry for the long wait :/

[jedwards1211]

No worries, I think it's inevitable on projects that one isn't paid to maintain...I've been even slower to keep up with some of my own