Investigate best way to add WebGL2 padding required by wgpu

[cart]

Bevy needs to add padding to some structs to support WebGL2. We are currently doing this, but the best way to do it is a bit unclear. Worth investigating!
Relevant discussion: #6643 (comment)

[nicopap]

Rust supports const-time panics. If all buffer bindings that require proper padding must be derived by a proc-macro we control, it should be possible to check the alignment of the struct (with mem::align_of::<T>()) and cause a panic at compile time. For example by defining a dummy const which evaluates to something that can panic.

I've already done this for a bevy-unrelated project, in an include-bytes-transmute macro.