Use rgb.rowBytes in overflow check

[radarhere]

Suggestions for python-pillow#5201

• When building libavif, at https://github.com/python-pillow/Pillow/actions/runs/12966407329/job/36170131250?pr=5201#step:5:2684 I can see

-- Could NOT find JPEG (missing: JPEG_LIBRARY JPEG_INCLUDE_DIR)

So let's build libjpeg first.

• TestFileAvif has @skip_unless_feature("avif"), which means that methods within that class don't also need the decorator.
• I'm confused by the overflow check.

row_bytes = rgb.width * avifRGBImagePixelSize(&rgb);

if (rgb.height > PY_SSIZE_T_MAX / row_bytes) {
  ...
}
...
size = rgb.rowBytes * rgb.height;

Shouldn't both the check and the multiplication use the same values? I've suggested that change here.

[fdintino]

libjpeg isn't actually used. libyuv has find_package(JPEG) in its CMakeLists because there are some jpeg tests and utilities, but those don't concern us. Still, there isn't any harm in ordering the build differently.

I agree regarding rgb.rowBytes. I can't say why I was calculating rowBytes here and not just using the avifRGBImage value.