The symmetric separate blur operation in the compositor is two times
slower in 4.4 compared to 4.3. On Linux, this only happens when Blender
is compiled with GCC, because Clang inlines a small function that GCC
doesn't.
To fix this, we specialize an if statement using templates to help GCC
inline the function. This results in a 3.5 times faster execution.
Pull Request: https://projects.blender.org/blender/blender/pulls/134336