Add PNG decode benchmark for narrow image widths

Add PNG decode benchmark for narrow image widths

Benchmark libpng row-by-row decoding where avail_out falls
below the 260-byte inflate_fast threshold. Uses a synthetic
gradient-with-noise pixel generator that produces deflate
token distributions representative of real photographs.
Also fix encode_png to use the passed width and height
instead of the hardcoded IMWIDTH and IMHEIGHT constants.