Don't use client bounding rect X/Y values

[Murreey]

Description

Replacing the use of Element.getBoundingClientRect's x and y values with left and top. They're always the same number.

Motivation and Context

On some old browsers, the x and y properties don't exist. We've polyfilled to fix it in most cases, but this change means the library works everywhere out the box again, without relying on polyfills.

How Has This Been Tested?

No functionality change, all tests are still passing. Will verify it on some broken devices.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[JohnPaulHarold]

thanks, was about to raise this issue. Saw this on YouView 2nd and 3rd gen Humax devices.