Add getPixel to OPC.pde

doc/processing_opc_client.md

examples/processing/grid24x8z_clouds/OPC.pde

examples/processing/grid24x8z_dot/OPC.pde

examples/processing/grid24x8z_flashy_rings/OPC.pde

examples/processing/grid24x8z_rings/OPC.pde

examples/processing/grid24x8z_rings_leapmotion/OPC.pde

examples/processing/grid24x8z_sequencer/OPC.pde

examples/processing/grid24x8z_text/OPC.pde

examples/processing/grid24x8z_waves/OPC.pde

examples/processing/grid8x8_dot/OPC.pde

examples/processing/grid8x8_orbits/OPC.pde

examples/processing/strip64_dot/OPC.pde

examples/processing/strip64_flames/OPC.pde

examples/processing/strip64_unmapped/OPC.pde

examples/processing/template/OPC.pde