Figure out how to handle BT.709-tagged images

[tirr-c]

libjxl linearizes BT.709-tagged images without ICC profile using BT.709 inverse OETF, whereas BT.709 recommends using BT.1886 EOTF when linearizing to display light. This leads to some inconsistencies:

• Say that we have an original image which is encoded in BT.709. Let's call this encoded signal A.
• Recommended way to linearize to display light is to use BT.1886 EOTF (which is pure gamma 2.4). Let's call this signal X.
  • Chromium uses sRGB EOTF (https://issues.chromium.org/issues/40539111).
• libjxl linearizes and encodes A using BT.709 inverse OETF (which is supposed to work on scene light). Let's call this signal Y.
• Inconsistencies happen when the encoded image is decoded back to PNG:
  • Most image viewers will display the original image as X.
  • Image viewers which prefers CICP tag will display the encoded image as X.
  • Image viewers which prefers ICC profile will display the encoded image as Y.
  • When the encoded image is decoded in sRGB color space, it will almost always be displayed as Y (or something similar), because it's what the image is actually encoded as.

Related to #318