Fix handling of top two pixel rows in be_blur_c

5dd56af2 replaced our implementation of \be with xy-VSFilter's.
This error is not present in the xy-VSFilter code; it was introduced
by accident in a merge that was later squashed as part of 5dd56af2.

Note: the new \be reads in but does not write out
the first and last row and column of pixels.
Remove dirty pixels from ASM be_blur output

A loop initializer was missing, so output started one row too early.
A loop condition check was missing, so output sometimes stopped
one column too late.

Also remove a couple of dead assignments.