Fixing up our pixel units is required for HIDPI support (devices and retina)

• need to have CSS pixels proportional to Au and different from device pixels
• layout is in terms of Au
• resize and the viewport window should be reported in CSS pixels (today, we are in device pixels!)
• we should convert from Au to device pixels after layout when we create the displaylist
• we have f32 everywhere right now, and need to start distinguishing them (especially because sometimes we have framebuffer size and sometimes window size, etc.)

• want small size fonts to be hinted well - need to know device resolution and the zoom level
• be careful because on mobile you want to avoid layout, so on zoom you want to avoid changing the font metrics and causing a reflow
• pcwalton: we are going to skip hinting because you don't need it on HIDPI (which is all mobile and retina displays).