Extend LOD support to work on billboards and particles

[supertuxkart-sourceforge-migration]

Author: auria

Extend LOD support to work on billboards and particles, examples of use :

• Torches should only burn when you're in range to see them, when you're far just skip them. Added difficulty : when there are multiple cameras, and none of them see the emitter, the calculations for particle positions could be entirely skipped
• We could want to have a model degrade into a billboard when seen from very far

Migrated-From: https://sourceforge.net/apps/trac/supertuxkart/ticket/141

[supertuxkart-sourceforge-migration]

Author: auria Updated LOD in r9663, r9664 is a step towards that

[supertuxkart-sourceforge-migration]

Author: auria This will actually be a bit more difficult than expected :( that's because atm LOD nodes are only supported in the ... section of scene.xml, while particle emitters are outside of this block (what is inside and outside the block are also loaded separately in the code)

[supertuxkart-sourceforge-migration]

Author: auria Added limited support for LOD on emitters in r9718. Still needed : 1) add to panel/exporter 2) make sure no memory is leaked

[supertuxkart-sourceforge-migration]

Author: auria Done in r9728.

The rest can be pushed back to later

[supertuxkart-sourceforge-migration]

Author: auria What's left :

• We could want to have a model degrade into a billboard when seen from very far