normalize the signature of the native interface
use pkg-config to select the available codec's
skip tests if the required codecs are missing
add seeking to the WriteBuffer
don't leak memory on a jpeg open error
basic support for tiff images
option to choose a couple of common tiff compression modes
support for webp images
remove failing tiff test
tests for the webp codec
option to set WebP alpha quality
document new api's, cleanup readme, bump version