Performance improvements in Stroke class

Adding support for pre allocated vertex and indices buffers in
Stroke.buildGeometry .

On my PC, this gives a performance boost from 2100 to 1200 ms on very
large test _lines (15000 StrokeVertex objects in _line, yielding about
450000 vertices) If this boost works on mobile devices, I don't know, I
only have a PC to run the code on.