Allow up to 4 elumens channels.

"spi fast" -> SPI_PF_AUTO.

src/elumens.cc

@@ -38,7 +38,7 @@ struct spistuff {
   Point Lens;	// offAxisLens
   int haschanxy, haschanwh;
   float chanxy[2], chanwh[2];
-  struct spichan chan[3];
+  struct spichan chan[4];
   int nchan;
   int refresh;
 };
@@ -76,7 +76,7 @@ void pp_spi_predraw( Fl_Gview *view, int passno ) {
     }
     if(spi.ctx == NULL) {
 	spi.ctx = spiInitialize( NULL, allpfchans |
-			(spi.fast ? SPI_PF_PBUFFER : SPI_PF_BACKBUFFER) );
+			(spi.fast ? SPI_PF_AUTO : SPI_PF_BACKBUFFER) );
     }
     spi.refresh = 0;
     if(spi.fast)