function: mirror_brain -- sagittal mirror brain

Flips current 4x4 transformation matrix by
inverting sign of its first row.  This may be
required since tkregister and paint read data
from raw BRIKs or bfloats, ignoring headers that
might indicate axis flips.

This button generates a flip with respect to the
sagittal plane if the MOVEABLE brain is already
approximately aligned.  If not, since there are
only two possible mirror-image states of a brain,
any remaining rotations can be corrected with the
TRANSLATE and ROTATE sliders.

Since it is difficult to determine the correct
left-right mirror-image flip of a brain, be sure
the TARGET brain is not flipped.  Tkregister
displays CORONAL and SAGITTAL images using
radiological convention (left=right).
