Change Qt::MiterJoin to Qt::SvgMiterJoin for context2d join

Qt::MiterJoin has different behavior with HTML5 context2d spec, fortunately Qt::SvgMiterJoin is the right one.

Change-Id: Ice50c6f92b244a4d39f984510f76146682b02f07
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>