improve resolving of optical sizes

[phi-gamma]

This concerns primarily large collections by Adobe, e.~g. Minion Pro, Garamond Premier Pro.

• test file: https://bitbucket.org/phg/lua-la-tex-tests/src/tip/ltx-fontspec-optical-size-3.tex
• script: http://tug.org/pipermail/lualatex-dev/2013-August/001603.html
• in addition, the development version of luaotfload-tool has support for real font request syntax, e.g. luaotfload-tool --find="Garamond Premier Pro/i/s=42" etc.

[phi-gamma]

New db format improves handling of sizes drastically.