threads: rework threaded fragment scheduling

Instead of creating fragment lists striped across available threads
uniformly in a round-robin fashion, just have the render threads iterate
across the shared fragments array using atomics.

This way non-uniform cost of rendering can be adapted to, provided the
module prepares the frame with sufficient fragment granularity.

In the ray tracer for example, it is quite common for some areas of the
screen to have lower complexity/cost than others.  The previous model
distributed the fragments uniformly across the threads with no ability for
underutilized threads to steal work from overutilized threads in the event
of non-uniform cost distributions.

Now no attempt to schedule work is made.  The render threads simply race
with eachother on a per-frame basis, atomically incrementing a shared
index into the frame's prepared fragemnts.  The fragment size itself
represents the atomic work unit.

A later commit will change the various renderers to prepare more/smaller
fragments where appropriate.  The ray tracer in particular needs more and
would probably further benefit from a tiling strategy, especially when
an acceleration data structure is introduced.

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