Cartoon shader improvements

Ping for code review or any comments from the community.

Cartoon shader improvements:
bugs.launchpad.net/panda3d/+bug/1221546

Bugfixes related to cartoon shading:
bugs.launchpad.net/panda3d/+bug/1214782
bugs.launchpad.net/panda3d/+bug/1219422

Please observe also that with both diffuse and specular quantized, the lighting obtains three “levels” like discussed (in 2010) by Anon and ninth in More on cartoon shading... even when a single threshold light ramp is used. Three levels can be obtained also by using a double threshold light ramp on the diffuse component only, but the visual styles produced by these two approaches are different.