Fixes Measure font-size setting issue(#2420)

dui phytolithrequested to merge
phytolithDui/inkscape:fix_2420_measure_fntsz into master

Fixes the Font Size issue for Measure tool #2420 (closed) .

The font size setting wasn't functional because it wasn't reflected on the Inkscape::CanvasItemBuffer object's context(buf->cr) where actual rendering is done.

Precision settings issue isn't fixed by this MR.

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