Release v0.21.3

Fixes a safety issue in the hdr decoder.

Release 0.22.1

- Fixes compilation without any enabled features

Release v0.22.0

- The required Rust version is now `1.34.2`.
- Note the website and blog: [image-rs.org][1] and [blog.image-rs.org][2]
- `PixelMut` now only on `ImageBuffer` and removed from `GenericImage`
  interface. Prefer iterating manually in the generic case.
- Replaced an unsafe interface in the hdr decoder with a safe variant.
- Support loading 2-bit BMP images
- Add method to save an `ImageBuffer`/`DynamicImage` with specified format
- Update tiff to `0.3` with a writer
- Update png to `0.15`, fixes reading of interlaced sub-byte pixels
- Always use custom struct for `ImageDecoder::Reader`
- Added `apply_without_alpha` and `map_without_alpha` to `Pixel` trait
- Pixel information now with associated constants instead of static methods
- Changed color structs to tuple types with single component. Improves
  ergonomics of destructuring assignment and construction.
- Add lifetime parameter on `ImageDecoder` trait.
- Remove unecessary `'static` bounds on affine operations
- Add function to retrieve image dimensions without loading full image
- Allow different image types in overlay and replace
- Iterators over rows of `ImageBuffer`, mutable variants

[1]: https://www.image-rs.org
[2]: https://blog.image-rs.org

Release v0.21.2

- Fixed a variety of crashes and opaque errors in webp
- Updated the png limits to be less restrictive
- Reworked even more `unsafe` operations into safe alternatives
- Derived Debug on FilterType and Deref on Pixel
- Removed a restriction on DXT to always require power of two dimensions
- Change the encoding of RGBA in bmp using bitfields
- Corrected various urls

Release v0.21.1

- A fairly important bugfix backport
- Fixed a potentially memory safety issue in the hdr and tiff decoders, see image-rs#885
- See [the full advisory](docs/2019-04-23-memory-unsafety.md) for an analysis
- Fixes `ImageBuffer` index calculation for very, very large images
- Fix some crashes while parsing specific incomplete pnm images
- Added comprehensive fuzzing for the pam image types