Merge branch 'blender-v2.90-release'

Eevee: do not rely on the SOCK_HIDE_VALUE flag for node group sockets.

When disconnecting links for defaulted node group inputs, recurse
into the nested node group nodes, instead of checking the socket
flag. Otherwise the behavior is confusing and differs from Cycles.

Differential Revision: https://developer.blender.org/D8455

Merge branch 'blender-v2.90-release'

Node Groups: expose the SOCK_HIDE_VALUE flag for node group inputs.

This flag specifies that even when the socket is not connected,
the node should not display the input field for the constant input
value. This is useful for inputs like Normal, which have special
handling for the missing input case and don't use a constant value.
Currently there is no way to change this flag from Python, and
through UI it can only be done by re-creating the socket.

This patch exposes the flag through RNA and UI, makes sure it
is properly updated when changed, and adds special handling to
ensure that it is correctly set when creating a node group from
a node set that includes reroute nodes.

Differential Revision: https://developer.blender.org/D8395

Node Groups: fix placement of the Output node when creating a group.

Instead of assuming the node width, compute the actual right bound.
Otherwise BSDF nodes tend to be too wide, and intersect Output.

Differential Revision: https://developer.blender.org/D8395

Drivers: add lerp and clamp functions to namespace.

Implementation of lerp without a function requires repeating one of
the arguments, which is not ideal. To avoid that, add a new function
to the driver namespace. In addition, provide a function for clamping
between 0 and 1 to support easy clamped lerp, and a smoothstep function
from GLSL that is somewhat related.

The function implementations are added to a new bl_math module.
As an aside, add the round function and two-argument log to the
pylike expression subset.

Differential Revision: https://developer.blender.org/D8205

Apply Modifier: support applying as shape key and keeping the modifier.

This can be useful to save the result of a cloth simulation as a
shape key without destroying the simulation, so it's possible to
e.g. re-run it to get other shapes, or simply use the new shape
key to start the simulation already in a draped state.

It also makes sense to allow applying as shape key even when the
mesh is shared, because the operation itself just adds a shape
key. To support this, split the apply operator into Apply and
Apply As Shapekey so that they can have different poll callbacks.

Differential Revision: https://developer.blender.org/D8173

Cloth: use the original vertex positions to build internal springs.

In order to ensure correct operation of the rest shape key feature,
it's necessary to create a temporary copy of the mesh with rest
vertex positions to run the BVH lookup on. If the rest shape key
isn't used, there is no need for additional overhead.

Cloth: correctly use the rest shape to compute the initial pressure volume.

Computing the original volume should use the rest positions of vertices,
like is done for initializing other parts of the simulation like springs.
Otherwise, features like rest shape key don't work correctly.

Fix ID reference counting asserts when freeing COW pose channels.

Take the do_id_user flag into account when freeing custom properties
attached to pose channels, so that pointer properties don't cause
assertion failures when DEG frees its COW instances.