Separate creation time & flush time behavior in GrDrawOpAtlas

This CL clarifies what is going on in the GrDrawOpAtlas and GrAtlasGlyphCache.

For the GrDrawOpAtlas:
  At creation time all the allowed pages are created (with their backing GrTextureProxies) but they aren't instantiated.

  The GrDrawOpAtlas::instantiate call is called in preFlushCB and allocates any pages known to be needed at the start of flush

  GrDrawOpAtlas::addToAtlas is called at flush time and, if a new page is activated, will instantiated it at that time.

  During compaction, an unused page will be deInstantiated but its Plots and backing GrTextureProxy will remain alive.

The GrAtlasGlyphCache reflects the changes to the GrDrawOpAtlas
  It now carries a GrProxyProvider for when it needs to create an atlas
  It passes in a GrResourceProvider* at flush time to allow instantiation.

  It does not, yet, allocate that GrDrawOpAtlases it might ever require.

Change-Id: I54909b7a3ba4bec2db5f1218f6a2a3a1636f66d6
Reviewed-on: https://skia-review.googlesource.com/108520
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
16 files changed