[css-fonts-4] all-small-caps vs unicameral scripts

[fantasai]

WebKit recently got a bug filed against our all-small-caps implementation complaining that we are synthesizing small-caps for unicameral scripts (Japanese, specifically). The reporter pointed to Firefox as having the right behavior. Opening this issue to ask a few questions, particularly to @jfkthame:

• Not synthesizing for small-caps is unquestionably the right behavior, but for all-small-caps, where the idea is to get all the letters and symbols to match, is this the right behavior? It would create distinct sizes for glyphs belonging to bicameral vs unicameral scripts.
• If yes, then we shouldn't synthesize for any unicameral scripts. What do we do for symbols and punctuation, then? Should they match the sizing of unicameral scripts (full size), or those of bicameral scripts (shrunk down)?