Clean up WebGPUVertexInputDescriptor

[litherum]

Three changes:

• As described in Relationship between setVertexBuffers() and setBindGroup() is unclear #145, inputSlot in the
  WebGPUVertexInputDescriptor should instead be implicitly derived from its
  position in the inputs field.
• The format of each attribute is already present in the shader. We should remove
  the redundant information.
• "Input" is not a very descriptive name. Instead, rename it to be "VertexBuffer".

[Kangz]

We need to keep the format because for example a shader taking vec2 as an input can have data stored at in many different ways:

• As two 8/16/32 bit integers that are converted to a float with the same value.
• As two 8/16/32 bit integers that are "normalized" to be turned in a float in the [0, 1] range.
• As two float.

[Kangz]

I'd be in favor of keeping this, so that applications can define "holes" in their vertex buffers, which would allow minimizing state setting between two pipeline using very similar but different vertex buffers (they could match on VB 0 to 5 and one of them having VB 6 and the other VB 7).

Actually having holes in interfaces like this is something I also wanted to bring up for the pipeline layout and color attachments as well. We already had at least on project prototyping on Dawn ask if we could have holes in the pipeline layout.

[kainino0x]

JavaScript arrays are sparse and heterogeneous, so they don't prevent us from having holes. With the change at [1], we could probably (if the IDL accepts it) have:

{ vertexBuffers: [buffer0, null, buffer2] }

or

const vertexBuffers = [];
vertexBuffers[0] = buffer0;
vertexBuffers[2] = buffer2;
{ vertexBuffers }

[magcius]

The name change is OK (I would bikeshed for "VertexInputBuffer" but no strong feelings on that). Strong 👎 on the other two changes for rationale discussed as before.

[kainino0x]

I think I like VertexInputBuffer or VertexInputIndex (or VertexInputSlot because I don't think it hurts to still make a distinction on the word "index"), because it matches with VertexInputDescriptor.

[kainino0x]

[1] This would probably have to be sequence<WebGPUVertexBufferDescriptor?>

[kainino0x]

JavaScript arrays are sparse and heterogeneous, so they don't prevent us from having holes. With the change at [1], we could probably (if the IDL accepts it) have:

{ vertexBuffers: [buffer0, null, buffer2] }

or

const vertexBuffers = [];
vertexBuffers[0] = buffer0;
vertexBuffers[2] = buffer2;
{ vertexBuffers }

[grorg]

This was discussed in the 10 Dec 2018 meeting

[grorg]

Resolution at the meeting: @litherum will split this into multiple PRs.

[litherum]

Moved to #151