Cycles: Refactor microfacet BSDFs to remove separate anisotropy code

Since the sampling and evaluation functions handle both cases anyways,
there's not really a point for keeping the distinction in the kernel,
so we might as well cut down the number of CLOSURE_BSDF_MICROFACETs a bit.

Differential Revision: https://developer.blender.org/D7736

Cycles: Add new Sky Texture method including direct sunlight

This commit adds a new model to the Sky Texture node, which is based on a
method by Nishita et al. and works by basically simulating volumetric
scattering in the atmosphere.

By making some approximations (such as only considering single scattering),
we get a fairly simple and fast simulation code that takes into account
Rayleigh and Mie scattering as well as Ozone absorption.

This code is used to precompute a 512x128 texture which is then looked up
during render time, and is fast enough to allow real-time tweaking in the
viewport.

Due to the nature of the simulation, it exposes several parameters that
allow for lots of flexibility in choosing the look and matching real-world
conditions (such as Air/Dust/Ozone density and altitude).

Additionally, the same volumetric approach can be used to compute absorption
of the direct sunlight, so the model also supports adding direct sunlight.
This makes it significantly easier to set up Sun+Sky illumination where
the direction, intensity and color of the sun actually matches the sky.

In order to support properly sampling the direct sun component, the commit
also adds logic for sampling a specific area to the kernel light sampling
code. This is combined with portal and background map sampling using MIS.

This sampling logic works for the common case of having one Sky texture
going into the Background shader, but if a custom input to the Vector
node is used or if there are multiple Sky textures, it falls back to using
only background map sampling (while automatically setting the resolution to
4096x2048 if auto resolution is used).

More infos and preview can be found here:
https://docs.google.com/document/d/1gQta0ygFWXTrl5Pmvl_nZRgUw0mWg0FJeRuNKS36m08/view

Underlying model, implementation and documentation by Marco (@nacioss).
Improvements, cleanup and sun sampling by @lukasstockner.

Differential Revision: https://developer.blender.org/D7896

Cycles: Fix CUDA compilation after cone sampling fix

Cycles: Fix uniform cone sampling

This code is currently only used for the Glossy Toon BSDF, but it's a generic
building block that might be used for other things in the future.

To see why the current code does not give a uniform distribution, consider that
it chooses both angles uniformly, but the smaller the angle from the center of
the cone is, the smaller the differential solid angle is (similar to how
sampling disks by choosing radius and phi uniformly does not work).

Differential Revision: https://developer.blender.org/D7948

Cycles: Refactor microfacet BSDFs to remove separate anisotropy code

Since the sampling and evaluation functions handle both cases anyways,
there's not really a point for keeping the distinction in the kernel,
so we might as well cut down the number of CLOSURE_BSDF_MICROFACETs a bit.

Differential Revision: https://developer.blender.org/D7736

Modifiers: Add option to directly specify a 2D transform for UVWarp

Currently the only option is to warp based on the transform of other
objects, which is inconvenient if you want to e.g. control it through
a driver - you need to set up a dummy object and go through that,
which is clunky and should be unneccessary.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6690

Freestyle: Add option for rendering Freestyle to a separate pass

This allows for more flexibility in Compositing compared to the
hardcoded alpha-over that is currently used.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6829

Merge branch 'blender-v2.82-release'

Fix unreported: Packing/unpacking errors don't mention tiled images

Thanks to @dfelinto for spotting this!

Fix T73713: Tiled textures (UDIM) getting lost when saving the .blend but not the textures

The issue here is simple, IMA_SRC_TILED was missing from a check for whether the image should
be saved.

Note that tiled images won't be automatically saved if they have never been saved before.
For single images this is handled by packing them, but packing of tiled images isn't
supported yet.

However, in that case the file closing dialog will at least show a warning now instead of
completely ignoring tiled images.