Optimize QSGCurveProcessor: solveOverlaps()

solveOverlaps() is currently O(n²), and half of it is even disabled by default, unless QT_QUICKSHAPES_WIP_CONCAVE_JOINT is set.

The straightforward solution is just using QSGCurveProcessor::findOverlappingCandidates() instead of comparing everything to everything.

In the long run, evaluate if solveOverlaps() and solveIntersections() can be combined, to avoid having to do the findOverlappingCandidates() work twice.