• do not use const for parameters passed by value
• rename "Use Viewport Proxies" to "Use Cycles Procedural"
• add a proper RNA callback to update the depsgraph when toggling "Use Cycles Procedural"
• rename "Always Add Modifier" to "Always Add Cache Reader"
• also add constraints when "Always Add Cache Reader" is enabled

Fix missing hair when the emitter is not shown.

The logic to synchronize hair particles and objects separately was broken, the particles would
only be synchronized if the emitter object is shown (b_instance.show_self()).

This makes it so hair particles are always synchronized again (if they are themselves shown).
As the Alembic importer does not create hair particles, those particles will be ones manually
added by the artists, so outside of the scope of the procedural.

Add an API to RenderEngine to signify support of Alembic procedurals,
rename "Cycles Procedural" to "Render (Engine) Procedural".

The API is only implemented by Cycles, but external render engines can
also implement it.

This required to pass the Scene to ModifierTypeInfo.dependsOnTime,
which in turn is used in BKE_cache_file_uses_render_procedural.

I couldn't find an explicit way of getting the RenderEngine from the
Scene, so a new RenderEngine is created from the RenderEngineType
referenced by name by the Scene. This will have to be reviewed, but I
don't know how.

Use a flag to detect support for Alembic procedurals.

• Remove dependency on D10196
• Only enable use_render_procedural if the active render engine supports Alembic procedurals. If the engine is Cycles, we also check that the experimental feature set is enabled.
• Add messages to the modifier's UI to inform about whether the engine has procedural support, and in case of Cycles, to enable experimental feature set.

Rebase on master.

Tag CacheFiles for an update in ED_render_engine_changed to ensure that
modifiers are recomputed or disconnected from the CacheFiles when switching
render engine. This solves some of the update issues.

Add an RNA function to Scene which is called from the Cycles addon when
modifying the feature set. This calls ED_render_engine_changed and ensures
that CacheFiles and dependant modifiers (and maybe other data) are updated
when switching between support and experimental feature set.

Added RE_engine_supports_alembic_procedural to check if the render engine
does support the procedural, with proper checks for the Cycles feature set.
This is also called from BKE_cachefile_uses_render_procedural as simply
checking the flag there did not properly detect that the render engine was
not supporting (anymore) the procedural.

I think this resolves the remaining update issues.

Added const qualifiers, however removed the ones for the Scene as
RNA_id_pointer_create (called in RE_engine_supports_alembic_procedural)
does not take a const ID. (Those were added by this patch anyway.)

Note that the boxes are visible for the selection overlay during Cycles
renders since the Overlay engine does not support the Alembic procedural.

• Rebase with master.
• Add some comment.
• Missing const in some dependsOnTime callback.
• Unnecessary cast.
• Use LISTBASE_FOREACH.

• Rebase with master.
• Remove const from function declaration

Add BKE_scene_uses_cycles_experimental_features.

Remove is_cycles parameter.

Add an enumeration to avoid using magic numbers for the enum value of the experimental feature set. This could perhaps use a string comparison but all the functions comparing enum identifiers string are taking a bContext.

Rebase with master, check that cache file type is Alembic when checking for procedural usage.