Fix for dynamic polylines with polyline dash material

[jasonbeverage]

This fixes an issue where dynamic polylines (rendered with the PolylineCollection) weren't properly defining the POLYLINE_DASH define in the vertex shader and causing rendering issues b/c the v_angle wasn't being computed properly.

You can see an example of the issue here: http://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=1b4c2cf989ab0304020fdb3f94374e93

This fix looks for v_angle in the material shader and then adds the POLYLINE_DASH define if it finds it.

[mramato]

Thanks @jasonbeverage. This probably fixes #5450, since using a CallbackProperty triggers PolylineCollection.

@emackey can you take a look at this since you reported #5450 originally. Thanks!

[jasonbeverage]

Yeah that should fix it. I didn't know that issue existed :)

[emackey]

Confirmed, this does fix #5450.

@bagnell Are you OK with keying off such a simple name here? Should v_angle be renamed to something more unique?

[pjcozzi]

Thanks @jasonbeverage! Can you please update CHANGES.md?

As @emackey suggested, would you also mind renaming v_angle to something like v_polylineAngle? Probably don't use the czm_ prefix.

[jasonbeverage]

Updated CHANGES.md and also changed v_angle to v_polylineAngle.

[pjcozzi]

Thanks again!