Currently, the OpenEXR reader can't read single-layer XYZ images as a
combined image. Single-view images will only read the X channel, while
multi-view images will interpret the Z as a depth image and the Y and Z
channels will be treated as float passes.
This patch allows the reading of XYZ channels as combined images. For
single-view images, we just extend the RGB-like channel names we match
to contain XYZ. For multi-view images we only treat the Z channel as a
Depth one if no X and Y channels exists, since the Z in this case is
part of the XYZ image.