Make gAMA and cHRM fallback optional for sRGB

[kornelski]

The sRGB chunk is sufficient on its own, so it'd be nice to be able to skip writing gAMA/cHRM fallback. I don't think there's value in adding the fallback by default. I assume that decoders too old to understand sRGB chunk are also too old to have good color profile support, so passing through unlabelled sRGB pixels may be safer for them.

The fallback is still possible if someone sets the fallback values explicitly, but I haven't bothered to increase API surface for that.

set_srgb was inconsistent with set_source_gamma/chromaticities, so this change is backwards compatible.

[fintelia]

The spec does say:

It is recommended that a PNG encoder that writes the sRGB chunk also write a gAMA chunk (and optionally a cHRM chunk) for compatibility with decoders that do not use the sRGB chunk.

That said, I'd be open to merging this since I agree with you that there probably isn't much software out there that understands gAMA/cHRM but not sRGB