[libpng17] Completed full working pngimage test program and added two test

scripts.  Fixed combination of ~alpha with shift. On read invert alpha,
processing occurred after shift processing, which causes the final values to be
outside the range that should be produced by the shift. Reversing the
order on read makes the two transforms work together correctly and mirrors
the order used on write.

tests/pngimage-quick

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec ./pngimage --log "${srcdir}/contrib/pngsuite/"*.png