test: Make surface demos reproducible by setting colormap to default at start of demo. * bicubic.m, interp2.m, interpn.m, griddata.m, image.m, axis.m, clabel.m, colorbar.m, contour.m, contourf.m, cylinder.m, ezcontour.m, ezcontourf.m, ezmesh.m, ezmeshc.m, ezsurf.m, ezsurfc.m, hold.m, pcolor.m, plotyy.m, quiver3.m, ribbon.m, shading.m, slice.m, sombrero.m, surf.m, surfc.m, surfnorm.m, trisurf.m: Set colormap to default at start of demos to make them reproducible.

--- a/scripts/plot/ribbon.m
+++ b/scripts/plot/ribbon.m
@@ -87,6 +87,7 @@
 
 %!demo
 %! clf;
+%! colormap ('default');
 %! [x, y, z] = sombrero ();
 %! [x, y] = meshgrid (x, y);
 %! ribbon (y, z);