`#include` this file to define the various bits and pieces needed to
support an extension. Now instead of having to change three places, we
now only need change one.
Also, sort the list of extensions. This list is starting to get a little
long and unruly, so before it gets completely out of hand, let's apply a
little semblance of order to it. The extensions are now arranged in the
following order: first, the Khronos-blessed extensions; then multivendor
extensions; then our (`MVK`) extensions; and finally other vendors'
extensions. These subgroups are sorted alphabetically, as Khronos'
master list is.
I'm not happy about the `MVK_EXTENSION_LAST` kluge. But, C++ apparently
doesn't let you stick a dangling comma on a constructor's initializer
list.
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