PNG: do not add iCCP if we have a gamma value
PNG: do not add iCCP if we have a gamma value

Sources with an ICC profile do not have a gamma value (we don’t try to
determine what it is), so they will still be written with an iCCP chunk.
Sources without an ICC profile, but with a gAMA chunk, will now have the
latter preserved.

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl