This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.
This patch has been generated by P1561 followed by `make format`.
Differential Revision: https://developer.blender.org/D8466
High quality emitters need to maintain state themselves. For example,
this it needs to remember when it spawned the last particle.
This is especially important when the birth rate is changing over time.
Otherwise, there will be very visible artifacts.
It is quite likely that other components of the simulation need their own
state as well. Therefore, I refactored the `SimulationState` type a bit,
to make it more extensible. Instead of using hardcoded type numbers, a
string is used to identify the state type. Also, instead of having switch
statements in many places, there is a new `SimulationStateType` that
encapsulates information about how a specific state is created/freed/copied/...
I removed the integration with the point cache for now, because it was
not used anyway in it's current state.
Object sockets work now, but only the new Object Transforms and the
Particle Mesh Emitter node use it. The emitter does not actually
use the mesh surface yet. Instead, new particles are just emitted around
the origin of the object.
Internally, handles to object data blocks are passed around in the network,
instead of raw object pointers. Using handles has a couple of benefits:
* The caller of the function has control over which handles can be resolved
and therefore limit access to specific data. The set of data blocks that
is accessed by a node tree should be known statically. This is necessary
for a proper integration with the dependency graph.
* When the pointer to an object changes (e.g. after restarting Blender),
all handles are still valid.
* When an object is deleted, the handle is invalidated without causing crashes.
* The handle is just an integer that can be stored per particle and can be cached easily.
The mapping between handles and their corresponding data blocks is
stored in the Simulation data block.
