Zero width borders break WR

[nc4rrillo]

While updating to the latest WR I discovered the following YAML:

root:
  items:
    - type: stacking-context
      bounds: [0, 0, 500, 500]
      items:
        - type: border
          bounds: [ 0, 0, 50, 50 ]
          width: [ 0, 0, 0, 0 ]
          border-type: normal
          style: [ solid, solid, solid, solid ]
          color: [ blue, blue, blue, blue ]

breaks WR with the following assertion:

thread 'WRRenderBackend#0' panicked at 'assertion failed: descriptor.size.width > 0 && descriptor.size.height > 0', webrender\src\texture_cache.rs:801:9

I think this has to do with moving borders to the brush code path and caching them in the texture cache.

Probably Gecko/Servo stop completely zero width borders from hitting WR at all. Should WR treat this as a no op rather than hitting an assertion?