Rotated text element misalignment in Agg

There is an alignment problem with rotated text elements in master when using the agg backend. In the svg and pdf output the two texts from the following example are aligned to the blue baselines as intended, whereas the png (agg) output doesn't provide the expected result.
```
plt.axhline(.5)
plt.axvline(.4)
plt.axvline(.6)
plt.xlim(0, 1)
plt.ylim(0, 1)
plt.text(.4, .5, "abcd", rotation=90, rotation_mode="anchor", va="baseline")
plt.text(.6, .5, "efgh", rotation=90, rotation_mode="anchor", va="baseline")
plt.savefig("roated_baseline.pdf")
plt.savefig("roated_baseline.png")
```
![test](https://f.cloud.github.com/assets/1819283/453871/4579c1be-b329-11e2-9830-43b40357571b.png)

A side effect from #1810 perhaps?