Add 1:1 (100% zoom) browser detection code

[Paul Sladen]

Currently the rendering may differ in terms of number of pixels from the textbox highlight, and therefore the two will be using different hint sets.

The form who keep track the zoom ratio and render this information into the image itself, or do something more intelligent (changing the pixel-dimensions of the canvas element proportionally to match; or put up a big banner than tells people to reset their font size using View->Zoom->Reset.

See http://tests.novemberborn.net/browsers/gecko/fullzoom/percentages.html for an idea of how to do this. Preferably check on something like an iPhone too where there is extreme zooming.