Cleanup: use float3 instead of float4 for shadow, since w is never used

Contributed by pembem22.

Differential Revision: https://developer.blender.org/D8947

Fix Cycles SSS incorrect rendering of zero radius in green channel

Thanks to pembem22 for finding the problem.

Ref D8949

Fix T79718: Eevee OpenVDB render error when frames miss part of the grids

Improved fix that handles the distinct cases of missing grids and zero
size grids.

Fix T80332: principle volume shader not working for world in Eevee

The handling of missing volume grids for the principled volume shader was
incomplete, different inputs need different default values.

Fix T79929: crashes with Cycles denoising on older Macs

This was a bug in OpenImageDenoise, upgrade to latest 1.2.3 which has the fix.
This in turn required a newer ispc version.

Differential Revision: https://developer.blender.org/D8892

Fix T78793, T79707: Cycles crash on macOS with older CPUs

Embree static libraries need additional linking flags to avoid SSE and
AVX symbols getting mixed up.

Fix OpenCL render error in large scenes

In scenes such as Cosmos Laundromat, there were memory allocations bigger than
2GB which would overflow.

Problem and solution found by AMD, thanks!

CMake: clean up setting of platform specific linker flags

Set flags directly on the target, and use common function for all cases.
This refactoring helps with the next commit for test executables.

Ref D8714

Tests: bundle tests for some modules in their own executables

The ffmpeg, guardedalloc and blenlib are quite isolated and putting them in
their own executable separate from blender_test is faster for development than
linking the entire blender_tests executable.

For Cycles, this also bundles all the unit tests into one executable.

Ref T79958

Differential Revision: https://developer.blender.org/D8714

CMake: clean up setting of platform specific linker flags

Set flags directly on the target, and use common function for all cases.