Stop binding glyph metrics callbacks to the first GlyphMetrics instance

[caolanm]

The function-local static in processGlyphs captures "this" once, so a second conversion in the same process runs the glyph callbacks against the first, already-deleted instance.

Description

seen in a debugging build that issues arise on reusing a this beyond its lifetime

Checklist:

• [X ] I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• [X ] I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation