Implement sBIT, tRNS chunks. Heavy-duty testing of copy_png.

"copy-all-png.t" copies everything in t/libpng/ and checks image data
same.

sBIT now implemented and working correctly. tRNS now implemented and
working correctly. Both parts documented, removed "Unimplemented
chunks" section from POD.

Changes updated, version to 0.36, released to CPAN.