Terrible performance on giant dotted and dashed border corners

[mrobinson]

When dotted and dashed border corners use very, very shallow angles, they create hundreds or thousands of dots and dashes. This leads to very bad performance. We should work to improve this situation. Some examples of web content that produces this issue are:

<span style="border-radius:50%;border:dashed;display:inline-block;width:115001px">xxx</span>

<span style="border-radius:50%;border:dotted;display:inline-block;width:115001px">xxx</span>

Wrench yaml that demonstrates this is:

---
root:
  items:
    -
      bounds: [0, 0, 1000, 1000]
      type: "stacking-context"
      items:
       -
         bounds: [16, 16, 115001, 50]
         "clip-rect": [16, 16, 115001, 50]
         type: border
         width: 20
         "border-type": normal
         color: black
         style: dashed
         radius: [57500, 25]

---
root:
  items:
    -
      bounds: [0, 0, 1000, 1000]
      type: "stacking-context"
      items:
       -
         bounds: [16, 16, 115001, 50]
         "clip-rect": [16, 16, 115001, 50]
         type: border
         width: 20
         "border-type": normal
         color: black
         style: dotted
         radius: [57500, 25]

[kvark]

Showing (as in - cargo run -- show xxx.yaml) one of the YAMLs provided doesn't render any primitives for me

[gw3583]

This is somewhat mitigated by #2795 - which stores dashed / dotted borders in the texture cache, to be re-used across display lists and frames. We should still apply some optimizations to the number of pixels touched by drawing the dashes though.

[gw3583]

@nical added a patch which calculates a much tighter bounding rect for each dash / dot to draw into, so I believe this is resolved now.