Output board dimensions in mm by default

Rather than the dimensions used in the Gerber files. The outlineGapFill option should also be in mm by default.

[mcous]

Yeah outlineGapFill has always been kind of a kludge, so I’m not really invested.

As for the units thing, I’m mostly approaching it from the standpoint of:

1. Not introducing breaking changes when an option will do
2. The units are involved all the way down the stack
   1. A given layer is going to be in. or mm. depending on the Gerber - Gerber coordinates are effectively mirrored in SVG land because the whole conversion is (basically) just a mapping function
   2. A unit difference between a layer and the board adds a transform attribute to that layer in the stackup
   3. Defaulting the board to mm without also defaulting the entire conversion process to mm would mean using Gerbers in inches creates board renders that are just a little bit harder to display every time - (Aside: defaulting the whole stack to mm might be good idea, but is a much larger project)

[mcous]

In hindsight, (and based on debugging some outlines with gaps) I think the switch to mm for outlineGapFill may have been a mistake. From what I can tell, outline gaps are often a function of resolution loss during gerber output from CAD. The resolution loss is caused by the fixed-decimal coordinate format of the Gerber file, which quantizes the locations that a segment’s endpoints can land on.

When outlineGapFill was “unit-less”, it was (almost) functioning as an epsilon, at its best saying “if two outline points land on adjacent discrete coordinate spots, consider them at the same point.” Specifying outlineGapFill as layer units was a confusing choice, though. Also, it wasn’t actually functioning as an epsilon because its value wasn’t tied to the coordinate format; its default value was simply selected to line up with the most common format.

Since the previous way of specify outlineGapFill was definitely more confusing than locking it to mm, I don’t want to revert it for 4.x. For the 5.x line, though, I think the most proper thing to do will be to change the option to an epsilon that is tied to the coordinate format. For example, 1.5 would actually do the “merge adjacent points in the matrix” thing.