Commits on Oct 26, 2017

1. Tidy up picture/brush code in preparation for box shadow optimizations. …

   There was some confusing code in the existing text shadow implementation
   that was making it difficult to optimize the box shadows to use sub-parts
   of a picture image. Specifically, in the cs_text_run and ps_line shader,
   we were fetching the owning Picture primitive. Instead, now the following
   conventions are observed:
   
   * When drawing *on* a picture, fetch the picture render task and the
     specific primitive only. This should have all information to draw
     on to a picture task.
   * When *compositing* a picture into the scene, fetch the picture
     render task for sampling information (e.g. UV rect, color mask),
     and the picture primitive for geometry information.
   
   In the second case, we don't currently need to fetch the picture
   primitive, since it's always just the entire primitive local rect.
   However, with the upcoming box shadow optimizations, this will
   typically be sub-parts or a 9-patch of the picture image.
   
   Importantly, this also decouples the geometry of the picture to be drawn
   from the geometry of the picture render task. This will allow box
   shadow optimizations such as rendering just a corner of the box
   shadow when uniform radii are present, and mirroring the blur task
   using brush segments.

   

   gw3583 committed Oct 26, 2017

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