make Resource a variant type

[jfirebaugh]

class FileSource : private util::noncopyable {
public:
    virtual std::unique_ptr<FileRequest> requestSource(const std::string& url, Callback) = 0;
    virtual std::unique_ptr<FileRequest> requestSprite(const std::string& url, Callback) = 0;
    virtual std::unique_ptr<FileRequest> requestGlyphs(const std::string& template, const std::string& fontstack, const GlyphRange&, Callback) = 0;
    virtual std::unique_ptr<FileRequest> requestTile(const std::string& template, const TileID&, Callback) = 0;
    // etc
};

This is necessary for offline: the OfflineFileSource needs access to actual z/x/y tile coordinates. It shouldn't try to guess them from the URL because there's no way to do that with 100% reliability.

[jfirebaugh]

Retitled: we should do this with a variant, not with a ton of virtual methods.