Use reasonable defaults for pole merge angles

Merge branch 'master' into cycles_panorama_experiments

Cycles: Fix pole merging for perspective camera

Merge branch 'master' into cycles_panorama_experiments

Fix compilation error after recent merge

Merge branch 'master' into cycles_panorama_experiments

Cycles: Use separate values for merge start and end angles

It's handy to merge eyes before reaching the zenith.

Cycles: Pole merging for spherical stereo

The idea of pole merge is to fade interocular distance after a certain
altitude to zero when altitude goes closer to a pole. This should prevent
annoyances looking up in the sky.

Not optimal or something, but sohuld be good enough for testing.

Merge branch 'master' into cycles_panorama_experiments

Cycles panorama experiment: Make tile sampling hack aware of resumable render

Merge branch 'master' into cycles_panorama_experiments

Merge branch 'master' into cycles_panorama_experiments

Cycles: Commit hack which allows to render less samples on the poles

This is more an experiment for now to see how it could improve render
times without loosing too much quality. Implementation is quite hacky
due to timing constraints here.

Now it is possible to control how much samples will be rendered on
the pole, so now regular Samples are controlling samples in the middle
of the frame, and those samples are falling off to the pole samples
using sine interpolation.

Limitations:

- Only works for final renders without progressive refine option.
- It is per-tile samples, so tile size should be small enough to
see an effect.

This is not something even nearly considered for master, more like
a research and test project.

Merge branch 'master' into cycles_panorama_experiments

Cycles: Experiment with using Lambert cylindrical equal-area projection

This commit implements new panorama and environment texture projection
which is called after a dude named Lambert:

https://en.wikipedia.org/wiki/Lambert_cylindrical_equal-area_projection

Advantage of this projection is that it doesn't waste sampling time next
to the poles (which is a huge issue for the equirectangular projection).

Disadvantage of this model is that there'll be some distortion on the
poles when mapping it back to the sphere.

This is partially worked around by using scaling magic so there'll be
a bit more samples added on the top/bottom of the image (which makes it
technically a Roosendaal-Sharybin mapping since that's what we've been
discussing in the studio).

Maybe we should be more aggressive here with clamping of the sine, but
let's start doing some real-world tests first.

Cycles: Experiment with native cubemap support in Cycles

This commit implements a native cubemap rendering in Cycles,
which is currently a research project with the following goals:

- Make it easier to render cubemaps for engines which supports
or expects them to be used.

- Intermediate projection to optimize rendering of equirectangular
camera.

Cube map layout is not currently standart but it's most efficient
from the resolution and storage point of views and it's currently:

+-------+-------+--------+
| Back | Right | Bottom |
+-------+-------+--------+
| Front | Left | Top |
+-------+-------+--------+

It's easy to re-map views in the future.

There are still issues to be solved, but it'll be nice to get
VR and panorama artists involved into feedback already. So the
limitations are:

- There's no auto-remapping to equirectangular map happening yet,
so the only way to see result is to put result to an environment
map with cubemap projection and either re-render or investigate
it in viewport.

- Projection formulas are not optimized by any mean. This is to
keep them totally obvious to make it easier to make further
tweaks.