Improve animation decoding API

[fintelia]

This PR changes the animation API from:

impl WebPDecoder {
    fn has_animation() -> bool;
    fn next_frame(...) -> Result<Option<u32>, Error>;  // Automatically resets the decoder `loop_count` times.
}

To:

impl WebPDecoder {
    fn is_animated() -> bool;
    fn num_frames() -> u32;
    fn loop_count() -> u16;
    fn loop_duration() -> u64;
    fn next_frame(...) -> Result<Option<u32>, Error>;
    fn reset_animation(); // Decoder must be manually reset
}