Make SkFunction copyable so it can go in containers.

This totally overhauls the implementation to use ordinary inheritance-based type erasure.  I give up for now getting my manual vtable shenanigans to work with MSVC.  Still those same "expected ; before ), also expected ) before ;" errors.

I added support for uninitialized SkFunctions and operator=(), because it was fairly straightforward with this implementation.

The main downside here is that I've removed the inline implementation.  All SkFunctions involve a heap allocation, even when just wrapping function pointers.

BUG=skia:

Review URL: https://codereview.chromium.org/1056673002

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