ColladaParser::ReadPrimitives: rename "a" to currentPrimitive
ColladaParser::ReadPrimitives: get rid of "idx" iterator.

We will be adding non-sequential access to the indices vector in one of the upcoming commits.
Extract function ColladaParser::CopyPrimitive
Move vertex copy operation into switch statement.

We will be adding special handling for Tristrips in an upcoming commit,
which unfortunately doesn't map to a common copy loop as it is.
Add calculation for nuber of Primitives for TriStrips
Add Function ColladaParser::ReadPrimTriStrips.

This does the index mangling for reading tristrips.
Whitespace changes
ColladaParser::ReadIndexData: don't rely on the specified number of primitives.

Especially TriStrips don't specify triangle count upfront, so one needs to
sum up the count of triangles read from all <p> tags in a tristrip collection.
For Polylists, manually compute the start offset outside CopyPrimitive.
Rename ColladaParser::CopyPrimitive to ::CopyVertex.

That's a bit closer to what the function actually does.
ColladaParser: don't arbitrarily limit number of per-vertex attributes.
ColladaParser::CopyVertex: don't make a local copy of the vertex' indices.

Instead, we address the relevant indices in the main vector directly by
adding the input offsets to the per-vertex base offset.
Tristripped version of cube.dae generated using openscenegraph.

Command line used:
OSG_OPTIMIZER="TRISTRIP_GEOMETRY" osgconv cube.dae cube_tristrips.dae
Merge pull request #414 from l337r007/collada-tristrips

Collada: tristrip support: helps a lot :-). Thanks for that!