Luatex: use scriptstyle sizes from the otf file if available

[d1saster]

So far only in XeTeX the otf-file parameters ScriptPercentScaleDown and ScriptScriptPercentScaleDown were honored by the loading routines. This commit patches \@@_declare_math_sizes in order to get the same behavior for luatex.

[wspr]

Thanks — I wonder if this behaviour changed at some point. I thought all the extended fontdimens were set in the same way in both engines, but apparently not.

I haven't completely decided whether it would be "cleaner" in the LuaTeX code to just set the fontdimens manually when the font is loaded so there's a conditional loading section and then no branch. OTOH maybe the indirection is a bit messy in the long term and the approach you've taken here is easier to maintain if there's ever more of a split between the XeTeX and LuaTeX code.