Fig bug 3335 (http://bugzilla.lyx.org/show_bug.cgi?id=3335)

Fig bug 3335 (http://bugzilla.lyx.org/show_bug.cgi?id=3335)

	* src/mathed/InsetMathScript.[Ch]
	(InsetMathScript::dy01): new, jointly account for both superscript
	and subscript according to the rules in Appendix G of the TeXbook.
	(InsetMathScript::dy0, InsetMathScript::dy1): set ascent and descent
	values using dy01 for non-limits super/subscripts.
	(InsetMathScript::metrics): account for the new super/subscript
	placement rules when setting ascent and descent values.

	* src/mathed/MathData.[Ch]
	(MathArray::metrics): cache parameters needed for super/subscript
	placement. When nucleus is not empty, set its ascent value to zero
	such that it is correctly computed later, otherwise ascent values
	less than that of an 'I' are not reported.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17471 a592a061-630c-0410-9148-cb99ea01b6c8