AssetPipeline now has a path tracer and optionally uses embree. For now,
the path tracer only sends out visibility rays and generates a simple
hit-or-miss monochrome bitmap. It splits the render target into tiles
and invokes a JobSystem task for each tile.
Filament developers can optionally install embree using homebrew or a
debian package. This avoids bloating third_party and increasing our
build time. If embree is not installed, a friendly run-time error will
occur when attempting to invoke the rendering functionality in
AssetPipeline.
The AO code is not yet implemented, but this PR sets up the
infrastructure, API, and camera rays.
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