Fix calculations when renderer size doesn't match canvas element size

[diarmidmackenzie]

This is a fix for #6.

It doesn't address this secondary issue...

It's straightforward to scale the rectangle, but this causes a different issue: the secondary camera views can "bleed" outside its bounds. This is caused both by the canvas interpolation and the fact that the bounds of the target div(s) may not line up with the rendered pixel grid.

...but it's not clear to me that's a problem in all cases, and this fix makes things considerably better, if not perfect.

In working on this, I also spotted another issue which this PR doesn't fix, tracked as #11.