QSvgRenderer::boundsOnElement incorrect when <text> node contains <tspan>

Example: <text font-size="12px"><tspan font-size"5px">qt svg bugs </tspan></text>

1) Height incorrect. QSvgRenderer::boundsOnElement calculates the bounds of the text element using text width of the `tspan` but faultly the text height of the `text` element that might have another font size.

2) Width of a `tspan` will also always be one character too wide.

Note, that the height of a `text` element is always equal to the line height, regardless of the text content. I consider this a feature.