Add support for circular layout barplots

[fedarko]

Continuation of #201, since the #293 PR only supports rectangular layouts.

[fedarko]

planned drawing algorithm for a single layer (for now only focusing on feature metadata layers, but generalizing this to stacked sample metadata barplots shouldn't be too crazy) --

• First off, figure out the colors and lengths assigned to each tip in this barplot layer. This isn't anything new.

• Find the maximum-radius-from-the-root node in the tree (or if this isn't the first layer, the maximum-radius bar in the previous layer). This radius, plus some constant "gap" space, will be the minimum radius for this layer: i.e. all of this barplot layer's bars will begin at this radius. (This is analogous to the prevLayerMaxX used for drawing barplots in the rect. layout, which is the leftmost point for all bars in a layer).

• There is a metric we can get by computing 2pi / # tips in the tree which I'm going to call cscf. This is analogous to the yrscf attribute used in the rectangular layout. Basically, this is the "maximum" angle we can give each individual tip's barplot wedge-thing without things starting to overlap. For a tree with two leaves this will be 2pi/2 = pi, for a tree with three leaves it'll be 2pi/3, etc. This only needs to be computed once for a given visualization (assuming the number of tips stays constant, i.e. if we add dynamic tree shearing or something in the future we'll need to recompute this).

• For each tip, we need to draw a wedge-ish thing starting at an angle of tip.angle - (cscf/2) and ending at an angle of tip.angle + (cscf/2) (with the inner radius for the wedge being set to maxRadius and the outer radius being set to maxRadius + tip.length_for_this_barplot_layer). The effect of this is that each tip's wedge should look "centered" on the tip.

  • We can approximate this by drawing a bunch of rectangles (aka pairs of triangles) throughout this space*, similarly to how collapsed clades in the circular layout are drawn, but it would be ideal to use shaders to draw this as a circle with the requested area filled in. (In either case, I think we'd need to convert between Polar / Cartesian coordinates to figure out where to draw things -- shouldn't be too much work.)

Here's a very rough diagram of what this would look like for a tiny tree with 5 leaves -- the max-radius node is circled:

* For small trees with just a handful of leaves, we'd need to draw lots of rectangles (maybe 15ish, like for arcs / collapsed clade wedges) per tip to prevent things from looking too jagged. For trees with more than a handful of leaves, maybe just 1-2ish rectangles will be fine.

[fedarko]

A working version of these is ready in this branch, although the code is a bit unpolished:

Currently, bars are approximated by drawing two rectangles (aka four triangles). Although things look smooth for the moving pictures tree, it's clearer what's going on when you use a tiny tree:

(Ideally we'd draw more rectangles in this case to smooth things out, but it totally works anyway :P)