Fixed and extended frustum tests
Activate Frustum Culling by default
Added some sanity checks for textures that might not be available at bind time.
Entirely restructured Xflow concerning ChannelNodes
Implemented new syntax to use Prototypes/dataflows via compute.

Use compute="dataflow['dataflowUrl']".
Can be used with filters, too:
compute="result = dataflow['dataflowUrl'](a,b,c,d,e,f)"
Added test cases for empty/missing arguments in Xflow.
Fixed xflow tests.
Implemented DataAdapter for new <dataflow> and <compute> node. Untested.
Implemented <dataflow> and <compute> node, removed any use of <proto> in tests.
Renamed prototype tests to dataflow tests. Added new complex test case.
Changed internal behavior of order mapping.

If an keep filter is applied with order mapping, the channels in the
resulting map have now the SAME order of the names in the order mapping.

This is important, so that the order of output parameters declared with the "out"
attribut of <dataflow> is preservered.
Xflow optimization: Forward request to subnode depending on name filter.

Example: Node A is the same as Node B, only adds one field of name "X"
If we request a dataset from Node A not containing the name "X",
the request will be forwarded to Node B.
Updated testcases.
Merge branch 'xflow-dataflow' into develop