GHOST: Support Unity taskbar progress API

Unity itself is deprecated, but the API is also supported by KDE and the GNOME Dock extension,
which means that it will be useful for a wide variety of distributions.

To get a progress bar, the system must have a blender.desktop file and libunity installed.

The need for libunity is annoying, but the only alternative would be to integrate a DBus library...

Reviewers: campbellbarton, brecht

Differential Revision: https://developer.blender.org/D3106

Fix T53567: Negative pixel values causing artifacts with denoising

Now negative color values are clamped to zero before the actual denoising.

Cycles: option to make background visible through glass transparent.

This can be enabled in the Film panel, with an option to control the
transmisison roughness below which glass becomes transparent.

Differential Revision: https://developer.blender.org/D2904

Fix T53567: Negative pixel values causing artifacts with denoising

Now negative color values are clamped to zero before the actual denoising.

Cycles: Fix constness for load_kernels in device_cpu.cpp

Cycles: Improve denoising speed on GPUs with small tile sizes

Previously, the NLM kernels would be launched once per offset with one thread per pixel.
However, with the smaller tile sizes that are now feasible, there wasn't enough work to fully occupy GPUs which results in a significant slowdown.

Therefore, the kernels are now launched in a single call that handles all offsets at once.
This has two downsides: Memory accesses to accumulating buffers are now atomic, and more importantly, the temporary memory now has to be allocated for every shift at once, increasing the required memory.
On the other hand, of course, the smaller tiles significantly reduce the size of the memory.

The main bottleneck right now is the construction of the transformation - there is nothing to be parallelized there, one thread per pixel is the maximum.
I tried to parallelize the SVD implementation by storing the matrix in shared memory and launching one block per pixel, but that wasn't really going anywhere.

To make the new code somewhat readable, the handling of rectangular regions was cleaned up a bit and commented, it should be easier to understand what's going on now.
Also, some variables have been renamed to make the difference between buffer width and stride more apparent, in addition to some general style cleanup.

Mikktspace: Speed up the merging of identical vertices

Previously, Mikktspace just bucketed the vertices based on one spatial coordinate and then ran full pairwise comparisons inside each bucket.
However, since models are three-dimensional, the bucketing has a massive false-positive rate, and since pairwise comparison is O(n^2), the merging process is very slow.

But, since we only care about exactly identical vertices, there is a much more efficient approach - we can just hash all values belonging to each vertex and form buckets based on the hash.
Since the hash has 32 bits and considers all values, false-positives are very unlikely - and since both hashing and the radixsort that's used for bucketing are O(n), both asymptotical and
real-world performance (as well as code complexity) are significantly improved.

Cycles: Add per-tile render time debug pass

Reviewers: sergey, brecht

Differential Revision: https://developer.blender.org/D2920

Cycles: Add Volume Direct and Volume Indirect passes for volume-scattered light

No color pass because it's hard to define what to use as color in a volume.

Reviewers: sergey, brecht

Differential Revision: https://developer.blender.org/D2903

Cycles: Refactor PassType from bitflag to index in order to allow for more passes