Geom. shader: Tractogram: Speed improvements
Geom. shader: Tractogram: Speed improvements

* Performing screen space culling so that no primitives are created if they're outside visible screen
* Using quads (rather than 6 vertices) when adjoining two line segments that are close to parallel (~ angle < 37 degrees)
* Means that we can nicely handle sharp corners (using 6 verts) but still reduce the total number of triangles outputed per draw call

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