Fix gl2d axis title positioning

[etpinard]

fixes#198

Now that we have image test support for gl2d plots, might as well fix a long-standing bug 🎉

This PR relies on work done in:

• Textcache textalign gl-vis/gl-plot2d#5
• Pass text align opts gl-vis/text-cache#2

[etpinard]

The new baselines from 6241fe1 were generated locally after npm linking patch in gl-plot2d and text-cache. They fill failed on CircleCI until the aforementioned patches are reviewed and published.