Triangle visitor (picking) ignores QGeometryRenderer::vertexCount

For BufferVisitor this was fixed as part of QTBUG-65590.

TrianglesVisitor does handle indexed drawing but still ignores vertexCount() from the QGeometryRenderer. This is bad in cases where drawing is restricted to a subset of a buffer by setting this value. Only the "simple" case (when QGeometryRenderer::vertexCount() is 0 and so indexAttribute->count() is used instead) is handled.