Add depthSlice to WGPURenderPassColorAttachment

[beaufortfrancois]

@kainino0x Please review

See https://gpuweb.github.io/gpuweb/#dom-gpurenderpasscolorattachment-depthslice

[kainino0x]

LGTM so far, but this needs a JS change too, here:

emscripten/src/library_webgpu.js

Line 1618 in a997567

return {

(EDIT: wrong code)

[kainino0x]

sorry, that code was wrong.

var depthSlice = {{{ gpu.makeGetU32('caPtr', C_STRUCTS.WGPURenderPassColorAttachment.depthSlice) }}};
{{{ gpu.convertSentinelToUndefined('depthSlice') }}}

        "depthSlice": depthSlice,

[beaufortfrancois]

Thank you @kainino0x. I've updated the PR with your suggestions.

[kainino0x]

LGTM