New stuff & fixes in Blender OSA filtering.

While investigating alternative filters (Mitchell), I found two small
errors in the Gauss code, it clipped wrong and multiplied wrong, causing
settings other than filter size 1.0 to not work properly.

Took the last-minute liberty to add more filter types in Blender too.
Also wrote an extensive log about how sampling & filtering in Blender
works.

http://www.blender3d.org/cms/Samples_and_Filtering.723.0.html

source/blender/src/transform_constraints.c

@@ -507,7 +507,6 @@ void setLocalConstraint(TransInfo *t, int mode, const char text[]) {
 	the orientation where %s is (logically).
 */
 void setUserConstraint(TransInfo *t, int mode, const char ftext[]) {
-	float mtx[3][3];
 	char text[40];
 	short twmode= (t->spacetype==SPACE_VIEW3D)? G.vd->twmode: V3D_MANIP_GLOBAL;