Support a wider stroke on text elements

[SophieRefshauge]

The max setting (1) on the stroke option on the text element is quite tiny - especially when you look at it on a mobile screen.

It would be awesome if this was more like the stroke setting in Photoshop - which can go way broader:

[raytranuk]

I have done some investigation and there are 2 main parts to improving the outlines:

1. New code path that splits rendering of the main text with the outline and uses uses stencil tests to avoid overlapping issues when outlines are thicker.
2. Fix an existing open issue in the multi-channel sdf tool that generates the text textures we use from type faces - the issue causes some regions to be incorrectly included in outlines.

[raytranuk]

The solution in my PR was too expensive to proceed with - however, if the WebGL extension that I proposed is accepted and implemented, I can update the PR to use this extension to efficiently render this (and other) text rendering effect(s).