Enforce the MIAF-mandated pixel aspect ratio of 1:1

[wantehchang]

@leo-barnes @y-guyon

MIAF ISO/IEC 23000-22:2019 says:

7.3.6.5 Pixel aspect ratio

This property shall be supported by the MIAF renderer and may be associated with any image item.

The default pixel aspect ratio of an image, in the absence of association with a PixelAspectRatioBox
property, is 1:1 (i.e. "square"). This value of pixel aspect ratio is the mandatory value for all images
mandated or conditionally mandated by this document, i.e. all images conformant with this document
shall have a pixel aspect ratio of 1:1.

libavif should enforce this requirement. Also, when encoding an AVIF image, libavif can omit the PixelAspectRatioBox property to save space.

[wantehchang]

HEIF ISO/IEC 23008-12:2022 says:

6.5.4.1 Definition

Box type: 'pasp'
Property type: Descriptive item property
Container: ItemPropertyContainerBox
Mandatory (per item): No
Quantity (per item): At most one

When no pixel aspect ratio 'pasp' descriptive item property is associated with an image item, readers
shall infer pixel aspect ratio 1:1 to apply for the image item.

[y-guyon]

when encoding an AVIF image, libavif can omit the PixelAspectRatioBox property to save space

By default it is not written.
You mean rejecting any other value than 1:1 for avifPixelAspectRatioBox when AVIF_TRANSFORM_PASP is set by the user, and never writing pasp?

[wantehchang]

Yes, that's what I meant.