When we use `getBoundingClientRect()`, we should account for borders.

[diarmidmackenzie]

When a screen element being rendered to has a border, these aren't allowed for in the render viewport calculations.

This means that rendering might be slightly off-center vs. what's expected, especially in cases where the element being rendered to has wide or asymmetric borders.

I have a fix for this, but I am concerned about the possible performance impact.