DBG_ASSERT->assert

Change-Id: Ic2933fc686d3bb030291756bd94127ee7583c833
tweak assert so comment appears in abort message

Change-Id: Ibf78e5cd1620f0b61cae030e3870be4a6f87e71d
Resolves: fdo#80160 PNG with 1-bit colormap only show black and white

Change-Id: If0c9950c18e6091fafea47954a4654db436a3a44