New feature; User definable Clipping Planes.

Press ALT+B in 3d window, draw a rect, and it becomes a clipping
volume of 4 planes. You then can rotate the view anyway you like.
Works for each 3d window individually.

Disable it with another ALT+B press.

Commit is huge because it had to change all selection code as well.
The user-clipping planes are in 'eye space', the other clipping
happens in projected 'viewport space'.

Nice to notice is that the 'x=3200' convention (to denote a coordinate
is clipped) now is a define. Define value is still a number though... but
we now can get up to screens of 12000 pixels without issues!

Known issue; here it refuses to draw the 'object centers' or Lamp icons
within the clipping region. Can't find any reason for it... however, we
might move to non-pixmaps for it anyway.

Testing might reveil numerous issues, will be standby for it.

Curious? Check this http://www.blender.org/bf/rt4.jpg

source/blender/src/transform.c

@@ -117,7 +117,7 @@ static void helpline(TransInfo *t, float *vec)
 	
 	getmouseco_areawin(mval);
 	project_float(vecrot, cent);	// no overflow in extreme cases
-	if(cent[0]!=3200.0f) {
+	if(cent[0]!=IS_CLIPPED) {
 		persp(PERSP_WIN);
 		
 		glDrawBuffer(GL_FRONT);