Output file is blank or missing some layers

[Woody363]

Minimal changes required, but submitting this to save others several days of frustration

I experienced an issue where the output file contained only the edge cuts and nothing else (sometimes not even edge cuts) when creating a large 500mm x 500mm pcb.

Solution was to reduce the dpi from the default 1200 as (i assume) something was timing out when calculating the layers. A warning should be produced in this case so the user knows what has happened.

I also experienced some layers not being created because the color of the object in Inkscape was wrong (e.g. yellow doesn't work for some reason?!) I'm unsure why this would be the case, is it bug, feature (or user error)?