The function calculates the offset to a subimage within the surface, in
units of surface samples.
All unit tests pass with `make check`. (Admittedly, though, there are
too few unit tests).
The plan all along was to eventualyl move isl out of the Vulkan
directory, because I intended i965 and anvil to share it.
A small problem I encountered when attempting to write unit tests for
isl precipitated the move. I discovered that it's easier to get isl
unit tests to build if I remove the extra, unneeded dependencies
injected by src/vulkan/Makefile.am. And the easiest way to remove those
unneeded dependencies is to move isl out of src/vulkan. (Unit tests come
in subsequent commits).
