[`COLR`v1] Relax requirement on not drawing unbounded glyphs

[behdad]

The COLR table spec currently has these two paragraphs under the boundedness section [1]:

First, it says glyphs must be bounded, essentially making any unbounded glyph invalid:

A valid color glyph definition must define a bounded region

However, it goes on to specify, with a must, that implementations must not render the color glyph if the graph is not bounded:

If a clip box is provided for a color glyph, the color glyph is bounded, and no inspection of the Paint graph is required to determine boundedness. If no clip box is defined for a color glyph, however, applications must confirm that the color glyph definition is bounded, and must not render the color glyph if the defining graph is not bounded.

This is inconvenient for the implementations, because it requires a boundedness computation on the glyph graph before any painting can be started. Since those glyphs are invalid anyway, this is unnecessary burden.

This would be akin to requiring that CFF(2) invalid charstrings must not draw anything.

I suggest relaxing the spec by removing the requirement, and just say if the graph is not bounded, the results are undefined.

I have checked three implementations: FreeType and Fontations do NOT implement the boundedness requirement. HarfBuzz does.

[1] https://learn.microsoft.com/en-us/typography/opentype/spec/colr#metrics-and-boundedness-of-color-glyphs-using-version-1-formats

[behdad]

cc @PeterCon @drott @dfrg @rsheeter

[drott]

I am supportive of such a change.

[dfrg]

This also seems reasonable to me.

[rsheeter]

+1