Evas font: fix width query for OT

Evas font: fix width query for OT

Fixes T4068.
Simply querying the last glyph to determine the width of the glyph sequence
won't always work, as OT can have negative offsets (adjusts the placement of a
specific glyph better).

The solution is to calculate the "max width" of some sequence that will
guarantee us proper width results. The worst solution would be to iterate on all
the glyphs and sum up the max width. This is a bit impractical. Instead, we will
inspect just the "cluster" of the last glyph, if one exists.

This should have no performance impact on trivial cases, and very little impact
on the others.

@fix