Fix T74680: Incorrect mixing in Glare node

The mixing function was designed to give correct results for Mix values of
-1, 0, and +1, but the behavior between these points was not linear. This is
unavoidable, because the function depends on both Mix and Mix^2 (by
multiplying value and mf) so they could not cancel out completely.

The new formula simply calculates the weighted sum without trying to invent
a smooth function.
Value for MixGlareOperation is now passed directly without scaling because
it is then easier to use.
Note that the previous formula performed max() twice for both input image
and the result, now there is just one max() per channel because the glare
input can't be negative.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D7138

Fix T74680: Incorrect mixing in Glare node

The mixing function was designed to give correct results for Mix values of
-1, 0, and +1, but the behavior between these points was not linear. This is
unavoidable, because the function depends on both Mix and Mix^2 (by
multiplying value and mf) so they could not cancel out completely.

The new formula simply calculates the weighted sum without trying to invent
a smooth function.
Value for MixGlareOperation is now passed directly without scaling because
it is then easier to use.
Note that the previous formula performed max() twice for both input image
and the result, now there is just one max() per channel because the glare
input can't be negative.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D7138

EEVEE: Fix sky zenith bug

Careless use of acos() in spherical coordinates transformation was
deteriorating the precision near zenith (and nadir) and producing
glitchy pixels (best seen in longer focal lengths).

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D8266

EEVEE: Implement the missing Sky texture

I'm not sure if the Sky was deliberately left out or was just waiting for a
better moment, but so many I was disappointed that Sky in EEVEE is
completely white.

There are already 2 implementations (osl and gpu) so this is the third one.
Looking at other cases it seems that we are not supposed to share sources
between cycles and the rest? So the new util_sky_model files are just
copies of what is already in cycles, except that the data file uses the RGB
variant of the Hosek/Wilkie model, because we output RGB anyway (but can be
easily changed to XYZ if desired - the results are nearly identical).
I am not sure if it is okay to pass 3*9 float values as 3 mat4 uniforms (I
wanted to use mat3 but it does not work).
Also, should I cache the sky model data between renders if the parameters
do not change?

Reviewed By: fclem, brecht

Differential Revision: https://developer.blender.org/D7108

Collections: preserve exclude flag of child collections when unexcluding

Excluding a collection also changes the exclude setting on all child collections
so that it is possible to selectively enable some children without the parent
being enabled.

This change makes it so that if you unexclude the parent, the exclude setting
of children are restored again instead of being permanently lost.

Original patch by Szymon with modifications by Brecht.

Differential Revision: https://developer.blender.org/D7016