Add contrib/examples/.clang-format, tailored to fit the existing code
as closely as possible. The end goal is to set up automatic formatting
for the entire libpng source tree. We're doing this experiment in this
subdirectory, for now.
Also make refactoring changes, as follows:
* Rewrite the preprocessor checks `#if PNG_FOO_SUPPORTED` to stop
compilation immediately, with a descriptive `#error` about what
needs to be supported.
* Rewrite and reflow comments, add braces and brackets, and make other
minor modifications that are suited for the clang-format'ed code.
