This is somewhat academic for now, as we explicitly ask for separate
layers and the scenarios where multi-plane images are required also use
complex formats that cannot be decomposed after the fact, but
nevertheless it is possible for us to consume simple multi-plane
images where there is one layer with n planes instead of n layers with
one plane each.
In these cases, we just treat the planes the same as we would if they
were each in a separate layer and everything works out.
It ought to be possible to make this work for OpenGL but I couldn't
wrap my head around how to provide the right DRM fourcc when
pretending a plane is a layer by itself. So I've left that
unimplemented.
Philip Langdale
89dfcf8286