Spatio-temporal filtering

variation works now, only with "infinite" samplers + timeouts around render time
some parameters might be scene-dependent
usage: check "useTemporalPathTracing" in Raytracing module
comment out #define USE_TEMPORAL in RayTracer/RayTracerCommon.h to ignore at compile time

Missing stuff:
- frame reprojection (acuumulation restarts every movement)
- spatial variation approximation (large bias for a few seconds: overblurring and initial brightness)
- subsampling/antialiasing (horrific things around edges for now)

Mysterious bugs:
- old color in tile remaining after rerender when zooming/middle clicking, bad synchronization?