EEVEE: Fix Missing GGX multi-scattering on Glass BSDF

Oversight that should have been in rB6f3c279d9e70

EEVEE: Add support for GGX Multi-scatter

Based on http://jcgt.org/published/0008/01/03/

This is a simple trick that does *not* have a huge performance impact but
does work pretty well. It just modifies the Fresnel term to account for
the multibounce energy loss (coloration).

However this makes the shader variations count double. To avoid this we
use a uniform and pass the multiscatter use flag inside the sign of f90.
This is a bit hacky but avoids many code duplication.

This uses the simplification proposed by McAuley in
A Journey Through Implementing Multiscattering BRDFs and Area Lights

This does not handle area light differently than the IBL case but that's
already an issue in current implementation.

This is related to T68460.

Reviewed By: brecht
Differential Revision: https://developer.blender.org/D8912

Fix T79557 EEVEE: Normalize in vector math node is not null vector safe

This add basic null safe handling for this operation.

GPUImmediate: Add system wide workaround for wide line

This makes wide line supported on MacOS and other implementation that
does not support wide line by default.

This workaround works for all Line types but only if using one of the 5
default shaders.

The workaround is completely isolated and invisible to the outside. It has
no side effect.

Note: This does not affect the GPUBatch drawing.

GPUShader: Fix wide line emulation with flat color interpolation.

This was causing flashing colors in the node editor grid.

This is because in some cases the flat color is only set on the provoking
vertex which is the last of the primitive by default.

Revert "Fix T79523 Paint Cursor: Wide line not supported on OSX"

This commit caused regression on some sculpt paint cursors. A better
approach is being worked on.

This reverts commit 6ade522f277fb74d4691973b7bb55840300043a2.

Merge branch 'master' into tmp-vulkan

# Conflicts:
# CMakeLists.txt

VK: Avoid asserts in debug builds.

GHOST: X11: Enable debug context for vulkan in offscreen contexts

GHOST: Vulkan: Rework support for MSVC

This removes designated initializer because not supported by MSVC for our
current C++ version.

Fixes some other compile errors and add usage of the `ContextVK` context in
the Win32 backend.