New Image Contour shape

[elblake]

I've made a new primitive for adding a contoured mesh that fits an image's transparency or mask.

I've spent some time coming up with a contouring algorithm to turn images directly into paths
when importing them into wings, now it works and I've made a new plugin to add images with
an automatically contoured mesh. "Monochrome Mask" is for making a shape from a
non-transparent image. "Transparent Image" is for making a shape from the alpha channel
of an image, and the image is set as the texture.

The code for making paths from images will be useful for other new commands too, so
I put the code in a separate file called trace_2d.erl in the e3d folder, I wasn't sure if it should be
called that or e3d_trace_2d.erl. I can make the change if needed.

NOTE: New Image Contour shape.