A couple of wee transform featurettes;

- center of rotation for camera in cameraview rotate has to remain the
  camera center itself, drawing the dashed helpline then doesn't work,
  since it's behind the camera clipplane. Just disabled that line.

- made MMB switch for cameraview grab to become quadratic, for a dolly
  this feels OK, and makes it possible to move in small and large scenes.

- restored SHIFT modifier for translation and scaling. This based on old
  convention that allowed precision editing on top of the transform you
  already applied before pressing SHIFT.
  Solved it with a new flag (T_SHIFT_MOD), since the G.qual cannot be
  used. Transform() innerloop has to detect the SHIFT event itself.
  Also coded it with storing the mouseposition while SHIFT event happened.
  Hope Martin can approve! :)

- Martin's last commit made Manipulator Translate not work, it passed on
  a zero translation to the constrainter, causing NaN's. Nicely catched the
  exception.

- Fixed 'Trackball' to accept number input too

source/blender/src/transform_constraints.c

@@ -203,21 +203,27 @@ static void axisProjection(TransInfo *t, float axis[3], float in[3], float out[3
 	/* For when view is parallel to constraint... will cause NaNs otherwise
 	   So we take vertical motion in 3D space and apply it to the
 	   constraint axis. Nice for camera grab + MMB */
-	if(n[0]*n[0] + n[1]*n[1] + n[2]*n[2] < 0.000001) {
+	if(n[0]*n[0] + n[1]*n[1] + n[2]*n[2] < 0.000001f) {
 		Projf(vec, in, t->viewinv[1]);
 		factor = Inpf(t->viewinv[1], vec) * 2.0f;
-
+		/* since camera distance is quite relative, use quadratic relationship. holding shift can compensate */
+		if(factor<0.0f) factor*= -factor;
+		else factor*= factor;
+		
 		VECCOPY(out, axis);
 		Normalise(out);
-		VecMulf(out, factor);
+		VecMulf(out, -factor);	/* -factor makes move down going backwards */
 	}
 	else {
-		Projf(vec, in, n);
-		factor = Normalise(vec);
-		factor /= Inpf(axis, vec);
+		// prevent division by zero, happens on constrainting without initial delta transform */
+		if(in[0]!=0.0f || in[1]!=0.0f || in[2]!=0.0) {
+			Projf(vec, in, n);
+			factor = Normalise(vec);
+			factor /= Inpf(axis, vec);
 
-		VecMulf(axis, factor);
-		VECCOPY(out, axis);
+			VecMulf(axis, factor);
+			VECCOPY(out, axis);
+		}
 	}
 }