grafx - Extensible Java Graphics Package Code

- added hook to do alpha manipulation (RasterFloatPixBuff).
- condensed a lot of code, and put it into new classes FloatPolyEdge and
FloatPolyClipper.
- cleaned up implementation of clear().
- fixed up color conversions between FloatColor and int.

my next goal is to fix any bugs that these changes may have created, and to
implement a FloatPolyEdge pool, so that 'new' would not have to be called for
every FloatPolyEdge; drawing many polygons would otherwise lead to rapid
memory allocation and de-allocation and re-allocation (new edges created all
the time).