[PATCH] Nested baseline-shift are not additive

According to the spec (http://www.w3.org/TR/SVG11/text.html#BaselineShiftProperty),
baseline-shift properties can nest and are additive but not inheritable.

the following example is expected to render text on the same line though the tspan is back on the baseline:

<text baseline-shift="10pt">the quick <tspan fill="brown">brown</tspan> fox</text>

the following example is expected to render the tspan within another tspan higher up compared to the baseline:

<text> using <tspan baseline-shift="super">super <tspan baseline-shift="5pt">super!</tspan> script</tspan></text>

Attached are testcases for the baseline-shift property