shape a stretchy glyph : step 1. is incorrect

From https://w3c.github.io/mathml-core/#algorithms-for-glyph-stretching

1. If there is not any MathGlyphConstruction table in the MathVariants.horizGlyphConstructionOffsets table (respectively MathVariants.vertGlyphConstructionOffsets table) for the given glyph the exit with failure.

But 1. is actually only required by steps 3. and 4. not by 2. I believe this whole list should instead be written the same way as https://w3c.github.io/mathml-core/#dfn-preferred-inline-size-of-a-glyph-stretched-along-the-block-axis :

1. If the glyph's advance width (respectively height) is at least T ...
2. If there is a MathGlyphConstruction table ...
   1. Browse the list of MathGlyphVariantRecord...
   2. If there is valid GlyphAssembly subtable then use the bounding ...
3. If none of the stretch option above allowed to cover the target ...

Then the returned success/failure does not make any sense and can be removed. Checking quickly, it seems it's only used in https://w3c.github.io/mathml-core/#layout-of-operators for two occurrences "If it is not possible to shape a stretchy glyph" but instead they should really check that a glyph exists for character c with the first available font.

Additionally, the two places "If there is valid GlyphAssembly subtable" should probably be more accurately stated as "if MathGlyphConstruction.glyphAssemblyOffset points to a valid GlyphAssembly subtable.".

Start with the glyph-stretching algorithm in the MathML Core specification, then compare it with the preferred inline-size algorithm and the two references in layout of operators. Update the ordering and wording for MathGlyphConstruction and GlyphAssembly checks, and ensure the success/failure behavior and glyph-existence checks are consistent.