Add a rewritten version of upng that decodes staefully line by line, and so
does not require a bitmap buffer for the output.  This compares to original
upng approach that needs heap allocations for the input, the whole output
and intermediate allocations.

Instead of buffers for input, decompression and output, it only allocates
2 x lines of RGBA pixels (ie, a few KB), and 32KB of decompressed data for
backward references in the decoder, and decodes as needed into the 2-line
buffer to produce line rasterized results.  For a 600px width PNG, this is
just 40KB heap for the duration.