Merge branch 'blender-v2.81-release'

Fix T70937: Cycles fails in viewport when rendering with OptiX

Was caused by D6068, which did not handle "MEM_PIXELS" memory
when not in background mode. Before that it always fell back to using
generic device memory, so restoring that behavior. In future this
should be changes to use OpenGL interop for optimal performance.

Cycles: Fix out of memory when rendering some scenes with OptiX that work with CUDA

The OptiX implementation wasn't trying to allocate memory on the host if device allocation failed, while the CUDA implementation did. This copies the implementation over to OptiX to remedy that.

Differential Revision: https://developer.blender.org/D6068

Fix T70268: Render failures with Vector pass active with OptiX in Cycles

Rendering would produce invalid results or crash if the Vector pass was active but motion blur was inactive. This caused the OptiX BVH to be built with motion (because objects reported motion available), but the pipeline to be built without motion support (since with disabled motion blur this is not in the list of requested features). The two are not compatible and therefore caused issues. This patch fixes that by not building the BVH with motion if motion blur is not active (which makes sense).

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5968

Fix "motion_blur" tests with OptiX in Cycles

Curves with motion blur produced wrong results with OptiX (T69801). This is because the AABBs for the motion steps were calculated from incorrect attribute data because the offset into the attribute data array was incorrect.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5961

Fix "denoise_animation" tests with OptiX in Cycles

The OptiX device only loads the denoising kernels when the "use_denoising" feature is active. This was not set by the calling code however and therefore they were never loaded and attempting to launch them failed (see T69801).

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5946

Fix memory leak after using OptiX in Cycles

The "optix_devices" array was not freed on exit, which caused a memory leak (see T69801).

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5944

Cycles: Fix undefined behavior which can causes crashes with a misaligned address error

Cycles casts a pointer from ShaderDataTinyStorage to ShaderData, these structs by default had different alignments however (the former was 1-byte aligned, the latter 16-byte). This caused undefined behavior on at least the CUDA platform. Forcing both structs to use the same alignment fixes this.

CUDA toolkits newer than 10.1 run into this because of a compiler optimization.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5883

Cycles: add Optix device backend

This uses hardware-accelerated raytracing on NVIDIA RTX graphics cards.

It is still currently experimental. Most features are supported, but a few
are still missing like baking, branched path tracing and using CPU memory.
https://wiki.blender.org/wiki/Reference/Release_Notes/2.81/Cycles#NVIDIA_RTX

For building with Optix support, the Optix SDK must be installed. See here for
build instructions:
https://wiki.blender.org/wiki/Building_Blender/CUDA

Differential Revision: https://developer.blender.org/D5363

Cycles: add Optix support in the kernel

This adds all the kernel side changes for the Optix backend.

Ref D5363