OSX: little logical refinement in cmake

RPF: muli-pass bilateral filtering code

RPF: first working bilateral filtering code

RPF: working of filtering code

Fixed silly error in new tiled contribution code, leading to banding.

Merged with tiled_contribution_test.

The Film class now support the notion of thread-independent "tiles" for adding contributions.
This means that differen threads may add contributions to different tiles in parallel.

For now the code assumes that a contribution overlaps at most two tiles (due to pixel filter
extent), and the tiles are implemented as horizontal slabs. Contributions added to a given tile
are clamped to the tile boundaries.

The contribution pool has been reworked, such that a single Buffer contains contributions for
a single tile. If a contribution straddles tile boundaries, it is added to the Buffer for both
tiles.

In addition the pool will try to splat a full Buffer immediately, instead of waiting until it
has exhausted the list of free Buffers. If another thread is busy splatting on that tile, the
full Buffer is queued and the current thread returns a free Buffer. This reduces the chances
that a thread must wait for another thread to splat.

Outlier rejection is made compatible with this tiled scheme by storing extra rows for the
overlapping area between tiles. It relies on the fact that straddling contributions are added to
both tiles. Thus if the center of a contribution is outside the current tile boundary, the outlier
rejection code will store it in these extra overlap rows.

A parameter "tilecount" has been added to "fleximage" which can be used to request a specific
tile count. Otherwise the number of logical cpus is used as a basis for determining the tile
count.In either case the actual tile count is adjusted to fit certain constraints and may be
less than the requested number.

Added proper handling of outlier rejection with tiled contributions.

Added V1_NORMAL and V1_BSDF debug filter modes

Working on RPF preprocessing code

Box filter now works (NaNs reported as a bug in plastic and substrate material)