In #4251 (Render To 3D texture), IIUC this is going to be allowed

So, should copyExternalImageToTexture allow copying to a slice of a 3d texture? Currently this is this disallowed. The destination texture must dimension='2d'

Given rendering to a slice of a 3d texture is now allowed should this restriction on copyExternalImageToTexture be removed?

Note: My use case: I had code that made canvases (1 per slice) and uploaded them as slices of cubemaps and slices of 2d-arrays. I needed to test something with 3d textures so I just passed the same code a 3d texture and then ran into this limitation.