Non-standard extension should be behind flag

[drott]

Compare #927

I would prefer if we kept not yet fully standardized extensions behind a flag to avoid breakage and being able to follow Chromium's release process.

[dfrg]

We can either feature gate this or just mask out the cubic bit on read with a comment that it should be re-enabled when the spec finalizes. I lean toward the latter unless we have a case where we really need to parse cubic glyf outlines.

@rsheeter any preference?

[drott]

I lean toward the latter [...]

Yes, let's move forward with that, could you do a PR? Thanks in advance.