Z-Epsilon is approximately the smallest Z value recognized by the graphics stack
to make a difference in Z positioning.
Z-Epsilon depends on the current projection matrix,
i.e. Z distance of the viewer.
z_epsilon = z * z / ( zNear * ( 1 << zBits ) - z );
We utilize Z-Epsilon as the smallest offset between the button background shape
and the label text.
We also utilize Z-Epsilon as the offset to elevate the active (selected) shape (add z-offset) and select (pick) in Z descending order (Bug 1454).
GraphUI Scene: Expose Z-Epsilon API for default and current PMVMatrixSetup values; Button add Z-Epsilon API for ctor and setLabelZOffset(..)
GraphUI Enhance: API doc; Scene/Button Z-offset and -epsilon; Push temp AffineTransform to local method; Simplify BaseButton setCorner(0) -> setPerp(); Protected abstract ctor