Curve stroker: Antialiasing artifacts

The curve stroker can get very visible artifacts due to the derivatives antialiasing which need looking into. One thing to explore, which is also a performance enhancement, is to support uniform scaling transforms by passing the size of a pixel to the shader rather than calculating it. It will not work with perspectives, rotations, etc. but for a fair number of use cases, this should give a speedup since it simplifies the fragment shader. It is also an approach that is compatible with older hardware that does not support the derivatives extension.