Introduce custom acceleration profile

The custom acceleration profile allow the user to define custom
acceleration functions for each movement type per device, giving
full control over accelerations behavior at different speeds.

This commit introduces 2 movement types which corresponds to the
2 profiles currently in use by libinput.

regular filter is Motion type.
constant filter is Fallback type.

This allows possible expansion of new movement types for the
different devices.

The custom pointer acceleration profile gives the user full control over the
acceleration behavior at different speeds.
The user needs to provide a custom acceleration function f(x) where
the x-axis is the velocity in native device units per microseconds
and the y-axis is a unitless factor multiplying the velocity.

The user should take into account the native device dpi and screen dpi in
order to achieve the desired behavior/feel of the acceleration.

The custom acceleration function is defined using n points which are spaced
uniformly along the x-axis, starting from 0 and continuing in constant steps.
There by the points defining the custom function are:
(0 * step, f[0]), (1 * step, f[1]), ..., ((n-1) * step, f[n-1])
where f is a list of n unitless values defining the acceleration
factor for each velocity.
When a velocity value does not lie exactly on those points, a linear
interpolation of the two closest points will be calculated.
When a velocity value is greater than the max point defined, a linear
extrapolation of the two biggest points will be calculated.

This commit introduces a new variadic function to the API called:
libinput_device_config_accel_custom_set_points which allows the
user to provide multiple acceleration functions for each movement
type for a device. This support the possible expansion of movement
types disccused above.

Signed-off-by: Yinon Burgansky <>
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