Add a PPI property to basic_image_surface and a way to query the PPI for basic_output_surface/basic_unmanaged_output_surface

[mikebmcl]

This is needed to properly support point size sizing for text rendering and other PPI-specific sizing (if added). Need to mull over the proper API since output surfaces are scaled up by default currently, which would introduce fuzzy typography.

Introduce strong types for pixels and physical distance and overload on them? I know that graphics like this seem to be inch based but I would say we make them meter-based so std::ratio<1, 1> would mean meters.

So density can be expressed in negative powers, right?