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/surfc.m
+++ b/scripts/plot/surfc.m
@@ -77,11 +77,13 @@
 
 %!demo
 %! clf;
+%! colormap ('default');
 %! [~,~,Z] = peaks ();
 %! surfc (Z);
 
 %!demo
 %! clf;
+%! colormap ('default');
 %! [~,~,Z] = sombrero ();
 %! [Fx,Fy] = gradient (Z);
 %! surfc (Z, Fx+Fy);
@@ -89,6 +91,7 @@
 
 %!demo
 %! clf;
+%! colormap ('default');
 %! [X,Y,Z] = sombrero ();
 %! [~,Fy] = gradient (Z);
 %! surfc (X,Y,Z,Fy);