Commits on Apr 13, 2018

1. Change drop-shadows to be drawn via a single primitive. …

   Previously, drop-shadows were a Picture, with two brush image
   primitives, each referencing the picture (one for the content,
   one for the shadow with an offset).
   
   Although conceptually reasonable, this exposes some problems
   with the way the current prim_store visibility pass works.
   Specifically, we can end up processing the picture more than
   once. In general this is an inefficiency but doesn't affect
   correctness, but it breaks some assumptions once the content
   of the drop-shadow element is an item in the render task cache.
   
   In the future, this problem should disappear as we refactor
   how the visibility pass operates, but for now the simplest solution
   is to draw the drop-shadow with a single primitive. To make this
   work, there are a couple of hacks in this patch, which
   push an extra brush image primitive into the GPU cache data.
   
   The drop shadows still have an issue with the shadow disappearing
   if the content of the drop shadow is off-screen, however this is
   no worse than the existing code. Doing it this way actually makes
   it easier to fix this is the future too.

   

   gw3583 committed Apr 13, 2018

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