Add support for custom filter shaders

[fabmax]

That would indeed be quite useful. However, I think a Java passthrough of the filter shader will have pretty bad performance (the filter shader can be called an awful lot of times), so I haven't done that until now. It might be useful anyway.

[fabmax]

I gave it a try and it works better than expected. It's available in the current 0.4.16-SNAPSHOT release and in the dev branch.

Here is an example how to use it: PassThroughFilterShaderTest.

I tested the filter shader with 2000 colliding boxes and there is almost no difference in speed comapred to the native default filter shader.

A few notes:

• Setting up the custom filter shader also requires using the filterShaderData field in PxSceneDesc which would be painful from Java. Therefore this is done via PxTopLevelFunctions.setupPassThroughFilterShader(sceneDesc, shader); instead of sceneDesc.setFilterShader(...);
• The native filter shader callback has an additional output parameter pairFlags, which is difficult to emulate in Java. My solution is that the Java filter shader implementation has to call the method setOutputPairFlags(flags);
• The Java filter shader method gets the filterData parameters as individual ints instead of PxFilterData objects. This makes the method signature a bit messy but access to the individual words does not require additional JNI interaction and therefore is much faster.
• Also for performance, all flags are represented by primitive ints

[EddeDev]

Thanks! :D