Fix memory corruption caused by GlyphExtVertexSerializer

[embeddedt]

The Iris 1.7.x implementation of GlyphExtVertexSerializer performs out-of-bounds memory reads & writes when extending the vertex data. This leads to unpredictable behavior including consistent JVM native crashes on some platforms.

There are several problems in the original code:

• The quad view is set up using the extended format's stride, but reads from the source buffer which contains data aligned for the non-extended format.
• The quad view is set up using a pointer that points to the end of the last vertex, but the logic inside expects the pointer to point at the start of the last vertex.

While investigating this problem, I determined the bug was quietly fixed upstream in this Iris commit for 1.8.0-beta.1. The fix does not appear to have been backported to any 1.7.x releases; as such, previous releases of Iris for Fabric are likely also affected.

I have confirmed, through testing, that a certain class of JVM crashes appear to be resolved by this change.

[dima-dencep]

Thanks ❤️