Mat - here is my first pass at sorting out the frame trim issue

As we were discussing last night, here is my first pass at fixing the frame trim issue.

I've broken the commits up so you can see each one. Basically this now works perfectly for me when using the Canvas Renderer, either with or without trimmed atlases, scaled, anchored, rotated, etc. They all seem to render properly, but the returned frame dimensions are still accurate and not influenced by the trim values (as is how it should be).

Please have a look at what I've done (it's fundamentally very simple really) and if you think it's ok then please can you do the same for the WebGL renderer?