Add support for tiled images and the UDIM naming scheme

This patch contains the work that I did during my week at the Code Quest - adding support for tiled images to Blender.

With this patch, images now contain a list of tiles. By default, this just contains one tile, but if the source type is set to Tiled, the user can add additional tiles. When acquiring an ImBuf, the tile to be loaded is specified in the ImageUser.
Therefore, code that is not yet aware of tiles will just access the default tile as usual.

The filenames of the additional tiles are derived from the original filename according to the UDIM naming scheme - the filename contains an index that is calculated as (1001 + 10*<y coordinate of the tile> + <x coordinate of the tile>), where the x coordinate never goes above 9.
Internally, the various tiles are stored in a cache just like sequences. When acquired for the first time, the code will try to load the corresponding file from disk. Alternatively, a new operator can be used to initialize the tile similar to the New Image operator.

The following features are supported so far:
- Automatic detection and loading of all tiles when opening the first tile (1001)
- Saving all tiles
- Adding and removing tiles
- Filling tiles with generated images
- Drawing all tiles in the Image Editor
- Viewing a tiled grid even if no image is selected
- Rendering tiled images in Eevee
- Rendering tiled images in Cycles (in SVM mode)
- Automatically skipping loading of unused tiles in Cycles
- 2D texture painting (also across tiles)
- 3D texture painting (also across tiles, only limitation: individual faces can not cross tile borders)
- Assigning custom labels to individual tiles (drawn in the Image Editor instead of the ID)
- Different resolutions between tiles

There still are some missing features that will be added later (see T72390):
- Workbench engine support
- Packing/Unpacking support
- Baking support
- Cycles OSL support
- many other Blender features that rely on images

Thanks to Brecht for the review and to all who tested the intermediate versions!

Differential Revision: https://developer.blender.org/D3509

Cycles: perform clamping per light contribution instead of whole path

With upcoming light group passes, for them to sum up correctly to the combined
pass the clamping must be more fine grained.

This also has the advantage that if one light is particularly noisy, it does
not diminish the contribution from other lights which do not need as much
clamping.

Clamp values on existing scenes will need to be tweaked to get similar results,
there is no automatic conversion possible which would give the same results as
before.

Implemented by Lukas, with tweaks by Brecht.

Part of D4837

Cycles: support for custom shader AOVs

Custom render passes are added in the Shader AOVs panel in the view layer
settings, with a name and data type. In shader nodes, an AOV Output node
is then used to output either a value or color to the pass.

Arbitrary names can be used for these passes, as long as they don't conflict
with built-in passes that are enabled. The AOV Output node can be used in both
material and world shader nodes.

Implemented by Lukas, with tweaks by Brecht.

Differential Revision: https://developer.blender.org/D4837

Cycles: Scale denoising albedo contribution of Principled BSDFs according to average fresnel

The Principled BSDF uses Microfacet closures that include a fresnel term,
which are a special case since their weight tends to be near white even
if their average contribution is fairly low.

The sample weight is scaled by the average fresnel weight to account for
this, but the denoising albedo still used the unscaled weight.

This was fine for the original denoiser, but apparently OIDN can't handle
the resulting albedo pass well. Therefore, this commit adds the described
scaling to the albedo pass contribution as well.

This problem was described in T69770.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6289

Squashed UDIM patch (D3509) another time

Apparently the buildbot process changed and it builds the latest branch commit now.

Revert "Squashed UDIM patch (D3509) once again"

This reverts commit 4e90f53dbcf0220f2daa25bb8d1b389bee1bb8de.

Squashed UDIM patch (D3509) once again

Merge branch 'master' into experimental-build

Fix T71250: ID user decrement error with ID properties in ViewLayer

Credit for the original patch goes to @scorpion81.

Fix T71250: ID user decrement error with ID properties in ViewLayer

Credit for the original patch goes to @scorpion81.