Another update for the Sunflow Exporter

I've just finished up the next update to the exporter.

Changes in version 0.18 are:
- the script now checks the screen resolution, and makes the rollout shorter for lower resolutions
- legacy particle systems only export the particles visible in the viewport (doesn't apply to PFlow)
- adds camera motion blur
- adds diffuse texture map export
- adds bump map export
- adds UV export for meshes
- fixes a bug when exporting Strauss materials

Time to start thinking about what to put in next...