Disable fake iCCP in test.sng

Newer libpng versions perform sanity checks on input data, and error
out once they recognize that the fake iCCP in test.sng is bogus (it is
too short). So we comment out the iCCP block.
Fix broken base64 support (it is still fundamentally flawed)

This should not be called base64, see content of this commit.
HACK: Temporary hack to enable support for unknown chunks.

This should be redone to properly track the current 'location'.
For now, this enables us to pass most tests in the png test suite.