Fluid: Updated Mantaflow source files

This update introduces two improvements from the Mantaflow repository:

(1) Improved particle sampling:
- Liquid and secondary particles are sampled more predictably. With all parameters being equal, baked particles will be computed at the exact same position during every bake.
- Before, this was not guaranteed.

(2) Sparse grid caching:
- While saving grid data to disk, grids will from now on be saved in a sparse structure whenever possible (e.g. density, flame but not levelsets).
- With the sparse optimization grid cells with a value under the 'Empty Space' value (already present in domain settings) will not be cached.
- The main benefits of this optimization are: Smaller cache sizes and faster playback of simulation data in the viewport.
- This optimization works 'out-of-the-box'. There is no option in the UI to enable it.
- For now, only smoke simulation grids will take advantage of this optimization.

Merge branch 'blender-v2.91-release'

Fluid: Revert changes from T82488

Scaling of forces needs more work. Before making changes to them it would be nice to have a setup, that works physically correct across multiple modifiers (cloth, rigid bodies, fluid).

This will be a to do for 2.92.

Merge branch 'blender-v2.91-release'

Fluid: Removed clamp from force assignment

The clamp is too aggressive and results in forces being too weak.

Merge branch 'blender-v2.91-release'

Fix T82488: Mantaflow - force fields have very low influence compare to 2.90.1

Removed 0.2f factor when setting forces. Why this factor has been used when forces should only be clamped, nobody knows ..

Cleanup: Fluid engine API return types

Use bool return type where possible instead of int (the return values from fluid object are already boolean instead of int).

Also removed several if guards in API functions. If one of the arguments is in fact invalid / nullptr (should not happen though), it better to catch them directly where they failed and not silently escape them.

Cleanup: Use nullptr everywhere in fluid code

Switched from NULL to nullptr.

Cleanup: Fluid engine API return types

Use bool return type where possible instead of int (the return values from fluid object are already boolean instead of int).

Also removed several if guards in API functions. If one of the arguments is in fact invalid / nullptr (should not happen though), it better to catch them directly where they failed and not silently escape them.