Features:
- Added `Frame::from_grayscale_with_alpha` to efficiently generate a palette for `LumaA` pixels.
- Added `Decoder::current_frame_info` to re-query the `next_frame_info` without advancing the frame.

Enhancements:
- The palette for `Frame::from_rgba_speed` now handles multiple transparent
  colors, i.e. non-premultiplied alpha, by normalizing fully transparent pixels
  to one transparent entry. This leads to a better distributed palette.