Commits on Dec 22, 2015

1. Various fixes and refactorings to lay groundwork for future work, suc… …

   …h as GPU clipping, scroll layers with transforms etc.
   
   * Fix mix-blend-mode.
   * Use AABB tree per render target / scroll layer.
   * Change composite batching to create render target per tree layer.
   * Refactor stacking context flattening to clean up code.
   * Add debugging support to freelist.
   * Handle render target dependencies for composite batching.
   * Fix framebuffer readback during composite batching.
   * Fix various warnings.
   * Change render target allocation strategy (prep work for later).
   
   (This passes tests but causes some perf regressions that will be fixed in follow up patches).

   

   gw3583 committed Dec 22, 2015
   cdbea4a