GrainRedering: Some small improvements.

Squashed commit of the following:

commit bc88e5d4b43976a71cf50588f5d3b1b08cb1f522
Author: Nicola Vigano <nicola.vigano@esrf.fr>
Date:   Thu Sep 20 01:05:58 2012 +0200

    RenderGrains: fixed some GUI control and added help printing

Signed-off-by: Nicola Vigano <nicola.vigano@esrf.fr>

commit 98717fb1527ff4a1cbca3a106b12312611d539d9
Author: Nicola Vigano <nicola.vigano@esrf.fr>
Date:   Wed Sep 19 21:30:50 2012 +0200

    GrainRendering: speed up a little bit

Signed-off-by: Nicola Vigano <nicola.vigano@esrf.fr>

Signed-off-by: Nicola Vigano <nicola.vigano@esrf.fr>

git-svn-id: https://svn.code.sf.net/p/dct/code/trunk@780 4c865b51-4357-4376-afb4-474e03ccb993