Add a "Pixel Density" sub-panel to render output settings which
can be used to set the density (as pixels per inch for example).
This is then written to images that support pixel density.
Details:
- The scene has two values a PPM factor and a and base unit.
- The base unit defaults to pixels per inch as this is the most
common unit used.
- Unit presets for pixels per inch/centimeter/meter are included.
- The pixel density is stored in the render result & EXR cache.
- For non 1:1 aspect renders, the density increases on the axis
which looks "stretched", so the PPM will print the correct
aspect with non-square pixels.
Ref !127831
