In #4254 we allowed view to be 3d and added depthSlice to select which slice of view to render to.

However the change to allow 3d views in the "renderable texture views" algorithm also applies to resolveTarget. So the spec now says resolveTarget can be 3d, but there is no way to select a depth slice of it.

3d textures cannot be multisampled, but there is one scenario in which this matters: view is 2d multisampled, and resolveTarget is 3d. I do not know if this is actually possible, but the spec seems to allow it.

There are several options here depending on what the backends can support:

• Disallow resolveTarget from being 3d
• Similarly, require view and resolveTarget to have the same dimensionality, then this scenario doesn't work
  • (but it could be an issue later if 3d textures become multisample-able)
• Add a resolveTargetDepthSlice

cc @haoxli