Only treat glyph order as changed if it changed

#359 makes backend glyph jobs depend on ALL IR glyphs instead of the subset they actually rely on, specifically the IR for that glyph plus the IR for their components. The trick here is we don't initially know the components so we need to dynamically update read_access. I believe concretely when IR for a glyph finishes we can update it's Job's read access.

During incremental compilation we should only rebuild downstream (dependent) assets if their inputs _change_. Most notably, changing a glyph in a manner that doesn't alter the glyph order should not trigger rebuild of everything that relies on glyph order. Otoh, changing it in a manner that does, should. Both cases need tests.