[subset] Use vector instead of map for glyph bounds (#4232) * [subset] Use vector instead of map for glyph bounds Is faster. Part of https://github.com/harfbuzz/harfbuzz/issues/4231 * [subset] initialize bounds_vec value to 0xFFFFFFFF Some non-EMPTY glyph might have 0 bounds width/height --------- Co-authored-by: Qunxin Liu <qxliu@google.com>