Make attractors regular modifiers (#286)

Remove the built-in code and shader handling a fixed-size array of force
field attractors/repulsors, and related types `ForceFieldSource` and
`ForceFieldModifier`. Replace them with a new `ConformToSphereModifier`
describing a single attractor/repulsor source, which can be repeated to
add more sources. The new modifier is a regular update context modifier
without any specific built-in codepath, making its handling a lot easier
and cleaner, while also allowing any custom attraction force expression
as well as customizing other parameters of the source in real time.

Revamp the `force_field.rs` example to showcase this new modifier, and
add a dedicated parameter tweaking panel to observe the effect of each
parameter.

Fix a panic in rendering code when no effect is present. This looks like
a race condition; for now simply early out to prevent the panic.

```rust
effects_meta.dr_indirect_bind_group.as_ref().unwrap()
```

CHANGELOG.md

@@ -8,10 +8,21 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Added a new `ScreenSpaceSizeModifier` which negates the effect of perspective projection, and makes the particle's size a pixel size in screen space, instead of a Bevy world unit size. This replaces the hard-coded behavior previously available on the `SetSizeModifier`.
+- Added a new `ConformToSphereModifier` acting as an attractor applying a force toward a point (sphere center) to all particles in range, and making particles conform ("stick") to the sphere surface.
+
+### Changed
+
+- `ExprHandle` is now `#[repr(transparent)]`, which guarantees that `Option<ExprHandle>` has the same size as `ExprHandle` itself (4 bytes).
+- `EffectProperties::set_if_changed()` now returns the `Mut` variable it takes as input, to allow subsequent calls.
 
 ### Removed
 
 - Removed the `screen_space_size` field from the `SetSizeModifier`. Use the new `ScreenSpaceSizeModifier` to use a screen-space size.
+- Removed the built-in `ForceFieldSource` and associated `ForceFieldModifier`. Use the new `ConformToSphereModifer` instead. The behavior might change a bit as the conforming code is not strictly identical; use the `force_field.rs` example with the `examples_world_inspector` feature to tweak the parameters in real time and observe how they work and change the effect.
+
+### Fixed
+
+- Fixed a panic in rendering randomly occurring when no effect is present.
 
 ## [0.10.0] 2024-02-24
 

Cargo.toml

@@ -64,6 +64,8 @@ wgpu = "0.19.1"
 
 # For world inspector; required if "examples_world_inspector" is used.
 bevy-inspector-egui = "0.23"
+bevy_egui = "0.25"
+egui = "0.26"
 
 # For procedural texture generation in examples
 noise = "0.8"