Bump version to 2.0.19

Read #208 for details.

Unfortunately 2.0.17 was released on a branch. Later the branch was
removed and the 2.0.17 tag was placed on another commit and branch.

This resulted in various distros who pulled the tarball at different
times two have different jasper versions for 2.0.17.

https://repology.org/project/jasper/versions shows that some even have a
2.0.18.

To reduce all this confusion I will release 2.0.19 now.
With a clean changelog referencing what @MaxKellermann and @jubalh (me)
did on our fork at jasper-maint.

If we want to revert things later or improve the changelog this can be
easily done on master ontop of this.
But I feel we need this release to reduce the confusion and put the
project on a clear track again.

CMakeLists.txt

@@ -17,7 +17,7 @@ include(CheckCCompilerFlag)
 # The major, minor, and micro version numbers of the project.
 set(JAS_VERSION_MAJOR 2)
 set(JAS_VERSION_MINOR 0)
-set(JAS_VERSION_PATCH 16)
+set(JAS_VERSION_PATCH 19)
 
 # The project version.
 set(JAS_VERSION