[daip] CONVL and blanked images

[Eric Greisen]

Use SUBIM to copy the subimage - CONVL is probably just looking at the
header flag that says there are blanked pixels somewhere.  SUBIM will
correct that flag to match the subimage.

IMHEADER shows this flag.

CONVL does its operation with an FFT - blanked pixels are some 10**8
which would do wierd things to the FFT.  Substituting zero for blanks
sometimes is ok although that  may still make edges.

Eric Greisen