Faster Pixel Beads Photo Pane

Just like the [original](http://www.thingiverse.com/thing:287025), but 3 to 4 times faster. Generates the shape as a 2D surface and extrudes the entire surface at once for better performance. On my machine, 50x50 compiles are ~1h 30m, down from 6+ hours.

Also fixes an error in the default parameters which sometimes generated non-manifold STLs.

Instructions
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The linear extrude takes up a lot of time. This will disable the preview in the Customizer, so use Preview mode, then switch it to Compile before finishing.

You can only use the Customizer for 20x20 images. Much larger images are still too slow. This is mainly intended for people who want to download the OpenSCAD file and generate their own photo panes locally.

To compile locally, set the mode variable to Compile, use the [image array tool](http://customizer.makerbot.com/image_array) to generate the picture array (don't forget to invert colors), and change the width and height to your image size. Be aware