Added edge detection to img2scad.html

[amatulic]

Edge detection added. Also updated version.

Before:

After:

The edge detection uses a Sobel gradient filter, which requires a slightly pre-blurred image.

• Both Gaussian blur and Edge detection have a radius setting.
• If Gaussian blur radius is 0, then the Edge detect radius is used for internal Gaussian blurring.
• If Gaussian blur radius is >0, then Edge detect uses the already-blurred image, with a Sobel filter using the Edge detect radius. In this way various combinations of blurring and gradient filtering can be done.

The Sobel filter, fortunately, is also separable like the Gaussian filter into vertical and horizontal convolutions, which improves the execution speed over filtering with a 2D kernel of arbitrary size.