Fix T73938: Cycles Vertex Color wrong if no layer is specified

The node would render black in this case (but should use the
'active_render' layer choosen in the object data properties -- this is
now in line to how this is handled for e.g. UVs)

This introduces ATTR_STD_VERTEX_COLOR and uses this thoughout, if no
particular layer is specified in the node.

Maniphest Tasks: T73938

Differential Revision: https://developer.blender.org/D6887

Particle editmode: add mouse independent "Select Linked" operator

The current "Select Linked" operator works based on mouse position and
makes no sense to call from the menus and was removed in rBdd9dfadaac9b.

This patch adds an operator independent from mouse position that just
selects all keys to a corresponding point (and adds back menu entries,
adds keymap entry).

The original operator is renamed to 'select_linked_pick' internally
(this is now more in line to how "Select Linked" works for meshes,
curves etc)

Differential Revision: https://developer.blender.org/D6823

Fix T73774: Error in Paint 'Clone from Image/UV Map' panel

This panel is not for 2D paint, IMAGEPAINT_PROJECT_LAYER_CLONE is only
ever used in projection painting, not 2D painting.
Add a proper poll for this.

Maniphest Tasks: T73774

Differential Revision: https://developer.blender.org/D6836

remove "Select Linked" from the particle select and context menu

The operator in its current state is based on mouse position and doesnt
make sense to be called from a menu.
(In fact it should be called 'select_linked_pick' internally and a
separate 'select_linked' should be implemented similar to how "Select
Linked" works for meshes, curves etc -- see D6823 for this)

Differential Revision: https://developer.blender.org/D6822

Particles: remove unused sel_col and nosel_col from
CacheEditrPathsIterData and PTCacheEdit

Followup to 80cecee5b7c8.

ref D6725

Fix T68788: Particle Edit Mode Hair vertex doesn't follow vertex theme
color

Selection and grooming only ever happens on hair keys (vertices), so
makes sense to use that theme color.

Sidenote: `CacheEditrPathsIterData` and `PTCacheEdit` have `sel_col` and
`nosel_col`, these can go, will do in a separate commit.

Maniphest Tasks: T68788

Differential Revision: https://developer.blender.org/D6725

Fix T59271: Curves with Extrude still show curve

Compared to 2.79, this also has the benefit of actually showing the curve
again if you set both bevel start and end to 0.0 for example.

Maniphest Tasks: T59271

Differential Revision: https://developer.blender.org/D6168

install_deps.sh: correct clang/llvm download URLs

Fix T73681: Python exception adding Empty objects via `object_data_add`
with "Enter Edit Mode" enabled

It is valild to add Empties this way, but we cannot enter editmode on
those.

remove poll for OBJECT_OT_parent_clear

This works on selected_editable_objects, so no active object mandatory.

After deletion of the active object and using slection tools such as
e.g. box select, the lack of an active object would prevent this
operator from being called without good reason.

Also cannot think of any other existing poll that would really make
sense (Editor type, ...).

ref T68975

Maniphest Tasks: T68975

Differential Revision: https://developer.blender.org/D6773