1D read() is specified to support a level/lod parameter. The MSL
specification claims it needs to be 0 because "mipmaps are not supported
for 1D textures", but that restriction isn't documented for the
"mipmapLevelCount" property of MTLTextureDescriptor. Other APIs do
supported mipmapped 1D textures. Multi-sample textures aren't supported
by msl_ld(), so we don't need to worry about them.