Sky: Use Gauss-Laguerre quadrature for optical depth calculation

This allows to use fewer evaluations (30 msec down to 23 for me) while giving more accurate results (3x-10x less relative absolute error) compared to classic ray marching.

Not a massive difference, but meh, it's better. For Cycles the speedup doesn't really matter much, but I also have a patch for Eevee support.

I've also tried Gauss-Legendre and Gauss-Lobatto - the latter was always worse, while the former was slightly better at 2deg elevation but notably worse on 15deg.

Unfortunately the same approach can't be used for the integration along the primary ray, since there we also need the accumulated transmission so far at every integration point, not just the total result.

Differential Revision: https://developer.blender.org/D13521

Fix T93858: Zstd-compressed .blend files from external tools aren't recognized

The issue here was that after the seek table check, the underlying file wasn't
rewound to the start, so the code that checks for the BLENDER header
immediately reaches EOF and fails.

Since Blender always writes files with a seek table, this bug isn't triggered
by files saved in Blender itself. However, files compressed in external tools
generally don't have a seek table.

Fix Windows builds after Zstandard commits

Add support for Zstandard compression for .blend files

Compressing blendfiles can help save a lot of disk space, but the slowdown
while loading and saving is a major annoyance.
Currently Blender uses Zlib (aka gzip aka Deflate) for compression, but there
are now several more modern algorithms that outperform it in every way.

In this patch, I decided for Zstandard aka Zstd for several reasons:
- It is widely supported, both in other programs and libraries as well as in
general-purpose compression utilities on Unix
- It is extremely flexible - spanning several orders of magnitude of
compression speeds depending on the level setting.
- It is pretty much on the Pareto frontier for all of its configurations
(meaning that no other algorithm is both faster and more efficient).

One downside of course is that older versions of Blender will not be able to
read these files, but one can always just re-save them without compression or
decompress the file manually with an external tool.

The implementation here saves additional metadata into the compressed file in
order to allow for efficient seeking when loading. This is standard-compliant
and will be ignored by other tools that support Zstd.
If the metadata is not present (e.g. because you manually compressed a .blend
file with another tool), Blender will fall back to sequential reading.

Saving is multithreaded to improve performance. Loading is currently not
multithreaded since it's not easy to predict the access patterns of the
loading code when seeking is supported.
In the future, we might want to look into making this more predictable or
disabling seeking for the main .blend file, which would then allow for
multiple background threads that decompress data ahead of time.

The compression level was chosen to get sizes comparable to previous versions
at much higher speeds. In the future, this could be exposed as an option.

Reviewed By: campbellbarton, brecht, mont29

Differential Revision: https://developer.blender.org/D5799

Use Zstandard compression for the sequencer cache

Reviewed By: campbellbarton, brecht, mont29

Differential Revision: https://developer.blender.org/D5799

Refactor low-level blendfile reading into separate files

Instead of handling mmap, compression etc. all directly in readfile.c, refactor
the code to use a generic FileReader.
This makes it easier to add new compression methods or similar, and allows to
reuse the logic in other places (e.g. thumbnail reading).

Reviewed By: campbellbarton, brecht, mont29

Differential Revision: https://developer.blender.org/D5799

D5799 macOS platform fix attempt 2

D5799 macOS platform fix

D5799 Part 1: FileReader refactor

D5799 Part 2: ZStd support