djeedai / bevy_hanabi / 27791683731

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Fix property binding and indexing (#543)

Properties are bound in WGSL as `array<Properties>`, where `Properties`
is the dynamically-generated property struct derived from the effect
description.

Previously, property binding and indexing had a couple of issues:
- Unused alignment to storage buffer dynamic binding value provided by
the GPU device, which is only required when using dynamic binding, but
property buffers are whole-bound.
- Property offset was just wrong. It indexed in terms of `u32` count.

The issue is not trivial; because multiple effects have different
property structs, we cannot bind a single array for all of them. So when
we bind a struct as array, it has to be located inside the GPU buffer at
a multiple of its size, which is a per-effect value. That means that
when allocating storage for the properties of an effect, the allocation
needs to be aligned at a multiple of its size. The new `PropertyBuffer`
implementation guarantees this, allowing for mixed sizes to be allocated
inside a same GPU buffer.

Note that the alternative is to use a `u32` offset. That simplifies
allocations, but require defining properties in terms of `array<u32>` in
WGSL, and manually reading and bit-casting all properties one by one.
This makes the WGSL code very complex and unreadable.

The `spawn.rs` example has been updated to use 2 effects with
properties, showing that properties work past the first effect (this was
not working before this change).

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