Support dpi other than 96dpi

I guess V8 thinks we're on a screen with 96dpi. I think wkhtmltopdf provides an option to change this to eg 300dpi instead.

To reproduce:
- ctx.font = '9pt arial';
- open and compare to eg Photoshop text at 9pt
- notice it's much smaller

To (temporarily) fix:

pt = function(screen_pt) {
return screen_pt / 96 \* 300; // presumably?
}
ctx.font = pt(9) + 'pt arial';