Needed a bit of twisting in generic private func behind the imm_draw_circle
helpers, but think it's fine.
Note that this demonstrate yet another downside of new dashed shader
compared to olde line style: not only does it needs more complex setup,
and can only work with PRIM_LINES type of primitives, but it also
behaves totally wrong with chained short segments!
We really need to find a better way to do this effect at some point. :(
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