fix ClassCastException

PGraphicsDelegate/Zone need to extend from PGraphicsOpenGL so as to call
the proper methods
beginPickDraw: fill with correct color components

The results of the bitwise AND needs to be bitshifted to get the proper
component value in the range of 0-255
find and display pickColor errors

This should have never gone to the closest color, as any non-mapped
color means an error in the color drawn into the pickBuffer. The
previous behaviour only was a work-around to hide bugs