apis: release official apis.

apis: release official apis.

tvg picture is going to be released, we need the Saver as well.
Also, two stable periperal apis are released in v0.5

@APIs:
Result Picture::load(const char* data, uint32_t size, const std::string& mimeType, bool copy = false) noexcept;
CompositeMethod Paint::composite(const Paint** target) const noexcept;
Result Saver::save(std::unique_ptr<Paint> paint, const std::string& path, bool compress = true) noexcept;
Result Saver::sync() noexcept;
static std::unique_ptr<Saver> Saver::gen() noexcept;