Limit OpenType composite glyph recursion

[treeform]

Fixes #564.

Summary:

• Thread a bounded recursion depth through glyf composite glyph parsing.
• Raise PixieError when composite glyph references exceed the limit.
• Add a regression test that patches a Roboto glyph to reference itself.

Tests:

• nim r --hints:off --warnings:off tests\test_fonts.nim
• git diff --check