Fixing Panel Editing in AC3D

The X-Plane export plugin for AC3D doesn't handle panel textures very well.  The current plugin tries to identify cases where you have used your panel background as a texture - this queues it to generate ATTR_cockpit.  This scheme has a number of problems:

1. The search paths for the panel background are not up-to-date.  The plugin doesn't know about the new naming conventions or the cockpit_3d folder.
2. The scheme doesn't address panel regions at all - there is some support for them but it doesn't work well.
3. Most important: panel editing is not WYSIWYG.  Since you are using the panel background as your texture, you can't actually see where all the moving parts are!  Doh!

That last point is perhaps the most important one, and it is why, for the next version of the plugin, I am introducing panel previews.

Basically a panel preview is a screenshot of your panel with the instruments on it, sitting in your cockpit_3d/-PANELS- folder.  AC3D will recognize and use the panel preview when possible.  This will solve problems (1) and (3) - there will be only one naming convention for previews, and they will be screenshots of the panel in action, so you can texture with a preview of the instruments.

Plane-Maker 930 will contain a facility to generate panel previews; if you are using X-Plane 920, you can generate the preview manually by taking a screenshot in X-Plane.

For panel regions, we will have one preview file for each region (e.g. Panel_preview0.png, panel_preview1.png).  This addresses issue 2 - usage of the region previews will invoke ATTR_cockpit_region.

Finally,  I am moving the panel sub-region information from the preferences to the .ac file (hopefully) so that it will be saved with your plane.

Hopefully this will make a work-flow which is much simpler.  To make a 3-d cockpit you will simply pick "generate previews" in Plane-Maker, and then start using the previews as textures.