test: Remove compile-time option `SINGLE_ROWBUF_ALLOC` from pngtest.c

We have been running address-sanitized CI verifications for a while.
We can finally afford to simplify pngtest.c by removing a compile-time
option and the associated code branches that used to serve (only to a
limited extent) the purpose of bounds checking.

Also change the brief description of pngtest.c at the top of the file.
This is no longer just a simple test program.
1 file changed