Fix bounding rect of glyph runs in multi-line QTextLayout

When getting the glyph runs from a QTextLayout with multiple
lines, the glyph runs would be merged if possible, but not their
bounding rects. This was an oversight.

[ChangeLog][Text][QTextLayout] QTextLayout::glyphRuns() now returns
united bounding rects for glyph runs that are merged.

Change-Id: Ibbeaa99ecfc4e82e7965342efdae7c3c2b637343
Task-number: QTBUG-50715
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>