QTextDocumentLayout: Clipping issue when using selections

When using selections in QAbstractTextDocumentLayout::PaintContext the glyphs might get clipped wrongly on the left if the left-bearing of those characters is negative (e.g. when using Thai glyphs). The clipping is correct if no selection in QAbstractTextDocumentLayout::PaintContext is done.

Attached is a Qt-only sample application that shows the issue.

I've also attached a patch against the Qt-4.8 branch, but I'm not sure whether it is acceptable in regards to backward compatibility. I was confused that QTextLayout::draw() completely ignores the clipping if no selection is used but uses the quite restrictive clipping as soon as a selection is there.

In libmeegotouch the general approach was that we use the margins/borders to avoid having clipped glyphs with bearings/accents, so one part of the patch in QTextDocumentLayout does not substract the document-margin anymore.

Please let me know whether I should provide an "official" merge-request (with gerrit?) or whether the fix must be done completely different anyhow. Thanks.