Make QSvgRenderer::isValid() return false if the document is not valid SVG Tiny 1.2

It would be nice if QSvgRenderer::isValid() returned false if the document is not a valid subset of SVG Tiny 1.2, which is what Qt supports.

Currently isValid() returns true as long as the document could be read and is well formed, even though Qt may not be able to actually render it correctly.