Changes:
- Planar files can now be read with `Decoder::read_image_to_buffer` if the
  limits allow for such a large buffer size. Previously only the first plane
  was read and the decoder will attempt a fallback to this behavior if the
  limits do not allow all planes to be allocated.

Fixes:
- An off-by-one error in a `debug_assert` of `ValueBuffer` caused its use in
  `IfdDecoder::find_tag_buf` to panic outside release mode when the buffer
  exactly matched the required size.

Additions:
- The methods `ColorType::{num_samples, bit_depth}` are now public. This
  promises that we only have homogeneous color depths in this major version.