Helpful information for BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED

[kpreid]

Is your feature request related to a problem? Please describe.

1. Write a shader, uniform buffers, etc.
2. Try to run the program with the WebGL backend.
3. Get an error that tells you BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED is unsupported, and the pipeline name, and nothing else.

Describe the solution you'd like
Provide diagnostic information to help fix the alignment problem, such as:

• The fact that this error is provoked by the shader's code, not the size of CPU-provided buffers.
• The name or number of the binding that was the wrong size.
• The field layout of the struct type of that binding, if it is a struct.

All but the first of these improvements would require making the MissingDownlevelFlags error able to carry additional context, or some other kind of refactoring of the error types.

[rparrett]

I am a bevy user experiencing this error while migrating to a version of bevy that uses wgpu 0.13, and I would definitely love to have any sort of idea what's going on.

[cwfitzgerald]

The problem is that the size of your shader side struct must be aligned to size 16 - if it isn't, it needs to be padded out to be size 16

[gilescope]

One of your shader structs isn't aligned. Not going to tell you which one. You have to find the needle in the haystack. Sad face.

[cwfitzgerald]

@gilescope We should have the binding information available in the key of the hash map we're iterating through here, so a PR adding this information shouldn't be that difficult. https://github.com/gfx-rs/wgpu/blob/master/wgpu-core/src/device/mod.rs#L2857