Using Clipping PLanes in SUMA

NB: Use of clipping planes now requires that SUMA is in clipping plane
mode.  By default SUMA is not in clipping plane mode.  Shift-Ctrl-c
toggles in and out of clipping plane mode.

There may be up to six clipping planes.  No more clipping planes may
be added once there are six.

The clipping plane may be viewed as a rectangle, with Shift-c which
toggles the rectangle on and off.

The clipping plane may be moved forward and back with alt-scrollwheel.

The clipping plane is rotated, about the y-axis, with Alt-[left or
right arrow key], or about the x-axis with Alt-[up or down arrow
key].

The clipping plane is selected with Alt-[number of the plane].

The clipping plane is toggled on and off with Ctrl-[number of the
plane].

f reverses ("flips") the normal of the selected clipping plane.


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There can be up to 6 clipping planes, numbered:  1 2 3 4 5 6.  
Each has a separate color, being, in order       R G B C M Y.  
So, Plane 1 is Red, Plane 4 is Cyan, etc.

Below, "a number" can be in range [1-6].  There is additional
"super"-plane behavior (affecting all planes) for digits 7 and 0,
described below.

Here, "active" means a clipping plane is clipping, not just visible.

Key+mouse behavior
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Shift-Ctrl-c      : Toggles in and out of clipping plane mode

n                 : Successively add new clipping planes (in increasing 
                    numerical order), which are active as they appear (you 
                    might not see them to start, just their clipping effect)

[a number]        : Toggle the clipping behavior of numbered plane on/off 

Alt-[a number]    : Select the numbered clipping plane (e.g., so you can
                    rotate/translate it with other key presses).  If
                    the specified number > # planes, planes are added
                    up to the specified number.

7                 : Toggle all active clipping planes on or off 

0                 : If there are active clipping planes, turns all planes 
                    off and exits clipping plane mode.  Otherwise, turns all
                    clipping planes on.  On reentering clipping plane mode, 
                    turns all clipping planes on. 

Ctrl+f            : Flip clipping direction of selected plane

Shift+c           : Toggle view of all active clipping planes on/off

Alt/Cmd/Opt+[scrollwheel] 
                  : Translate a clipping plane in space

s                 : Scroll clipping plane inwards

Shift-s           : Scroll clipping plane outwards

Alt/Cmd/Opt+[up-down arrow key]      
                  : rotate clipping plane around x-axis

Alt/Cmd/Opt+[left-right arrow key]   
                  : rotate clipping plane around y-axis

