[REVIEW-3-5] fix for fdo#47299, calculating matrix formulas takes too long

Hey,

[1] improves the calculation time for matrix formulas by one
magnitude. We now just cache the calls to
mdds::mixed_type_matrix::size which brings the execution time back to
the 3-3 level. It does not only help for this test document but should
speed up all matrix formulas.

I think the patch is quite safe since it only changes from the direct
call to the cached result and updates the cached value for all
function calls changing the size.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=bbe088bc757fa120f50e4e104bdcd46c90f1feb1