Separate stub backends from other backends

Stub backends were a dependency of other backends to allow some code reuse.
However, that can cause conflicting symbols if multiple backends are linked
into the same binary, which should be possible.

To avoid this, move the shared code into the platform implementation, and
export those symbols so that backends can use them.  This adds some semi-public
platform-specific API that can only be used by backends included with pugl.
They are undocumented, subject to change at any time without a corresponding
version change, and may not be used by third parties (for example by custom
backends in an application).

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