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.

[diarmidmackenzie]

Branch fix-border-issue contains a fix for this that is functionally correct.

My concern is whether there are performance issues extracting the border information using getComputedStyles every frame. Also, though I hadn't noticed this before, in both old & new code, we create a new DomRect object every render, which also isn't aligned with performance best practice due to resulting garbage collection issues.

This might all be a non-issue, assuming:

• a relatively small number of secondary cameras...
• only applies when rendering to screen, which means we aren't in VR, and so very high FPS (72 or 90) is likely not required...

In any case, I want to merge #12 before creating a PR from this branch...