Get rid of multi-sample anti aliasing (MSAA)

[tuner]

WebGL's MSAA has a significant performance impact on large scatter plots like those in SegmentModel Spy. However, the actual scatter plot doesn't even need antialiasing, as it's using SDF-based shapes! With antialiasing, the full example data set when zoomed out is quite choppy with an M1 Mac but without antialiasing it's smooth.

Antialiasing is mostly needed for heatmaps that should have no gaps between the rectangles. An example of such a heatmap is the metadata in GenomeSpy App or the ideogram track. Without antialiasing, it looks ... badly aliased, particularly when animated. SDF-based edge-antialiasing kind of works, but the default blending mode causes visual artifacts and moiré effects.

On the other hand, MSAA is not perfect either, as it uses a limited number of samples. The following uses non-SDF rectangles, relying solely on MSAA. The banding is an artifact:

TODO:

• Test if additive blending (GL_FUNC_ADD) would do the trick
• If it does, add a blendMode mark property with an intelligent default value
• Also fix the endpoints of "rule" mark, which doesn't properly do SDF at the endpoints
• Set antialias: false for the WebGL context

If the above doesn't work, just make antialiasing optional, i.e., the embed method could have an options object with an antialias property.