As we started committing the command buffer on glyph cache flush, this invalidates the current encoder. We need to MTLTR_ValidateGlyphCache after the flush, not before.
There's no reason to maintain separate glyph cache invalidation logic for this singe case (which is a no-op in reality), so just free the cache instead.
MTLTR_ValidateGlyphCache
after the flush, not before.