Commits on Dec 4, 2017

1. Convert rectangle from prim to brush shader and support segments. …

   Add support for brushes to be drawn as a list of segments that
   are part of a larger primitive. Initially, these segments are
   nine-patches only, although they may be expanded in the future
   to work with generic segment rects.
   
   This allows large brush primitives to be split into a number
   of segments that can be split among the opaque and alpha passes,
   which means that large rectangles are often rendered with the
   inner rectangle as part of the opaque pass. Additionally, edge
   segments are rendered in the opaque pass when the primitive does
   not have a rotation or perspective transform applied.
   
   When a rectangle is split, the clip tasks are generated only
   where required for individual segments. This can significantly
   reduce the amount of allocated area for clip masks, and also
   the amount of GPU time spent drawing clip masks.
   
   Previously, rectangle splitting could only work on LocalClip
   items specified as part of the item, which meant that rectangles
   in Gecko would never be split. Now, we calculate splitting
   during prepare_prim_for_render, consulting the current clip chain
   for the primitive. This means that we get the benefits of rectangle
   splitting in Gecko, which doesn't use LocalClip API.
   
   Having rectangle splitting handled as segments also means that we
   don't add extra top-level primitives that take extra time to do
   visibility culling on, saving some CPU time.
   
   Future work:
    - Support more cases of primitive splitting.
    - Port images and gradients to be brushes so they automatically get splitting.

   

   gw3583 committed Dec 4, 2017

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