Support opening and saving L mode AVIF images with libavif >= 1.3.0

[radarhere]

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 of AVIF_RGB_FORMAT_RGB, to load it in L mode rather than RGB. Similarly, if the frame being saved is grayscale, using AVIF_RGB_FORMAT_GRAY simplifies encoding within libavif. However, AVIF_RGB_FORMAT_GRAY only 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.

[radarhere]

It turns out that Valgrind is raising an error - https://github.com/python-pillow/Pillow/actions/runs/23566085263/job/68628338690?pr=9471#step:5:6558

I've created AOMediaCodec/libavif#3139 to address it.

[radarhere]

My PR was merged, but their analysis of the problem is that while the uninitialized value is copied to a local variable, it isn't actually used.

So I suppose there's a choice here.
a) wait for a new release of libavif
b) wait for a new release of libavif, and restrict this feature to that new version or newer
c) avoid waiting and just silence the valgrind error on this test