make sure masters for global/glyph/layout metrics are rounded before computing deltas #2829

Job	Run time
resources/scripts/ots_test.sh	1m 18s
Rustfmt	1m 14s
Build GS and check the result	3m 8s
cargo check no std	1m 39s
Clippy lints	1m 28s
cargo test fea-rs	1m 5s
fontbe tests	58s
tests other than fontbe,fontc	2m 7s
fontc tests	1m 16s
	14m 13s

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