Commits on Nov 28, 2016

1. Introduce a glyph cache thread. …

   This is the first step towards making the glyph cache more efficient
   and also run asynchronously to allow rendering to continue if the
   glyph rasterizing becomes a bottleneck (e.g. during animated zooms).
   
   Previously, we collected the list of glyphs that were required for
   a frame, and ran a rayon loop to rasterize any that weren't in the
   glyph cache. However, this meant that the glyph rasterizion could
   not start until we knew the list of all required glyphs.
   
   Now, all requests for text run glyphs are sent immediately to the
   glyph cache thread. The glyph cache thread uses a worker thread
   pool to immediately begin rasterizing any uncached glyphs as soon
   as they are requested.
   
   In the future, the idea is that instead of waiting at the block()
   method for all pending glyphs to be rasterized, the backend thread
   will instead receive small numbers of glyphs as they are rasterized.
   This will allow the backend thread to make a decision that glyph
   rasterizing is taking too long for this frame, and that we should
   just draw a frame with the currently cached glyph sizes available.
   When this occurs, the glyph cache thread will signal that a new frame
   should be rendered when it completes rasterizing any pending glyphs.
   This will allow us to keep rendering at 60fps during zoom, even
   if the glyph rasterizing threads can't keep up (at a quality
   tradeoff of using lower resolution glyphs).

   

   gw3583 committed Nov 28, 2016

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