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.

[ghost]

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?