Support opening and saving L mode AVIF images for libavif >= 1.3.0#9471
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radarhere wants to merge 5 commits intopython-pillow:mainfrom
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Support opening and saving L mode AVIF images for libavif >= 1.3.0#9471radarhere wants to merge 5 commits intopython-pillow:mainfrom
radarhere wants to merge 5 commits intopython-pillow:mainfrom
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Resolves #9465
If an AVIF image is opened with YUV 4:0:0 subsampling, then it is grayscale, so we can use
AVIF_RGB_FORMAT_GRAY, instead ofAVIF_RGB_FORMAT_RGB, to load it in L mode rather than RGB. Similarly, if the frame being saved is grayscale, usingAVIF_RGB_FORMAT_GRAYsimplifies encoding within libavif. However,AVIF_RGB_FORMAT_GRAYonly become available in libavif 1.3.0, so this feature is only present for that version. As a side effect, this also means that if an RGB image is saved with 4:0:0 subsampling, it will be in L mode after reopening.Also, if all frames being saved to an AVIF image are grayscale, then if we default to 4:0:0 subsampling (instead of the current default of 4:2:0), then together, an L mode image can be saved and opened again in the same mode.