Ensure that `AREA` used by `IMG` is visible.

**Expectation:**
The function `isAreaVisible` used within `isVisible` should consider if `AREA` elements are outside of view/ coordinates of the `IMG` that uses the `AREA`.

**Actual:**
Currently the function `isAreaVisible` does not consider if referenced `AREA` is in view of the `IMG` that uses it.

**Motivation:**
If the coordinates sets an `AREA` that's bigger than the `IMG` it's mapped on, the `AREA` element will be visible to assistive technologies, but not to sighted users.

> Note: `isAreaVisible` is an enhancement to `isVisible` function as a part of of the rule `identical-links-same-purpose`. See PR & comment - https://github.com/dequelabs/axe-core/pull/1649#discussion_r335436527