config.h includes were missing in a few files, including input.c, the
lack of which caused the X11 backend to segfault instantly due to not
having an xkbcommon context.
Signed-off-by: Daniel Stone <daniel@fooishbar.org>
The weston_spring is a very flexible and powerful mecanhism for driving
animations. However, it can be a little difficult to tame, but this
little helper can plot the response of the spring to a set of initial
parameters and makes it easy to tune and tweak the spring behavior.
