Disable Constraint and Keep Transform

A 'Disable and Keep Transform' button for constraints was added. This
allows animators to disable a constraint without moving the constrained
object/bone, making it easier to toggle constriants on and off without
any visual consequence. Typical usage would be a character picking up an
object (enable 'Copy Transform' constraint) and placing it somewhere
else (disable the constraint).

Note that there could still be movement when there are muliple
constraints active. For example, when using this constraint stack

- #1: Copy Transform from Empty.001
- #2: Copy Rotation from Empty.002

and disabling constraint #2, constraint #1 is still active and will
still modify the visual transform of the object. According to our
in-house animators, this is expected behaviour.

Reviewers: campbellbarton, dfelinto, sergey

Reviewed By: campbellbarton

Subscribers: brecht

Tags: #animation

Differential Revision: https://developer.blender.org/D4677

Fix T61935 Crash when importing Alembic from Meshroom

The Alembic importer can now deal with XForm-less objects. Apparently
MeshLab exports point clouds without parent transformation matrix (XForm in
Alembic-speak). We shouldn't warn about this, but handle it properly
instead.

Alembic export: always consider metaballs animated

The metaball 'mesh' is animated whenever any of its components are animated.
However, it's not easily possible to visit all its components (AFAIK it
requires a full scan of all objects in the scene and comparing names), so
due to the blobby nature of the metaballs I now simply assume they'll be
animated.

This fixes an issue where a metaball was considered static when the base
ball was static. For example, MBall static but MBall.001 animated would
be considered 'not animated'.

Fix T62664: Exporting Metaballs as Alembic crashes Blender

The `AbcMetaballWriter` now subclasses `AbcGenericMeshWriter` instead of
wrapping an `AbcMeshWriter`. `AbcGenericMeshWriter` was created for this
purpose (writing non-mesh objects as mesh to Alembic) and performs the work
in a cleaner and, more importantly, not crashing way.

Alembic export: free depsgraph after use

Fix segfault when audio system cannot be initialised

`PyInit_aud()` can return `NULL` for variour reasons. When that happens,
we shouldn't continue.

Cleanup: return early in BLI_path_frame_get

Instead of making the entire body of the function conditional, it now
returns early, unindenting the entire function and preventing the reader
from searching for a non-existent `else` clause.

No semantic changes.

Fix BLI_path_frame_strip

The `BLI_path_frame_strip` function was completely broken, unless the
number of digits in the sequence number was the same as the length of
the extension. In other words, it would work fine for `file.0001.abc` (4
digit `0001` and 4 char `.abc`), but other combinations would truncate
to the shortest (`file.001.abc` would become `file.###.ab` and
`file.00001.a` would become `file.##.a`). The dependency between the
sequence number and the file extension is now removed.

The behaviour has changed a little bit in the case where there are no
numbers in the filename. Previously, `path="filename.abc"` would result
in `path="filename.abc"` and `ext=""`, but now it results in
`path="filename"` and `ext=".abc"`. This way `ext` always contains the
extension, and the behaviour is consistent regardless of whether there
were any numbers found.

Furthermore, I've removed the `bool set_frame_char` parameter, because
it was unclear, probably also buggy, and most importantly, never used.

I've also added a unit test for the `BLI_path_frame_strip` function.

Use the same string for DNA enum and CLI argument for EXR image format setting

The `--render-format` CLI option takes `EXR` and `MULTILAYER`, whereas
the DNA image format render setting uses `OPEN_EXR` and
`OPEN_EXR_MULTILAYER`. This commit adds the DNA values to the CLI
argument, so that it is possible to take the selected value from DNA and
pass it as-is to the CLI. This is used in Flamenco, for example.

The `OPEN_EXR` and `OPEN_EXR_MULTILAYER` are now aliases, so both the
existing and the new options keep working.

Reviewers: campbellbarton, brecht

Reviewed By: brecht

Subscribers: fsiddi

Tags: #bf_blender_2.8

Differential Revision: https://developer.blender.org/D4502

Fix crash reloading images used in compositor node

Similar to what Brecht committed in
38f0d5f75af7d2f30156f4008dc5bc8d8394fbf3.