Implement a touchpad scroll distance property

To be used for touchpads and continuous (i.e. button-based scrolling).

libinput provides us with pixel data for finger-based and button-based
scrolling but the X server does support this - XI2.1 smooth scrolling is
merely centered around a logical scroll click (defined as "increment"), with
smooth scrolling being a fraction of that increment. For example, in the old
synaptics driver that value was in device-specific units and thus different
for every device.

The increment is a constant value set in the ScrollClass and cannot be changed
at device runtime. So we simply initialize with a random default (15, because
that works well for wheels) and then scale our pixel delta in to that range.

With the default value, a 15 pixel movement would result in a logical scroll
click, if the distance is set to 30 the users has to move 30 pixels to trigger
that scroll click. Pixel here being defined as the deltas that libinput
provides to us.

From the client's perspective nothing changes, the increment is still the
same.

Range checks are quite restrictive, this option is supposed to improve
usability, not as a workaround around other bugs.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

man/libinput.man

@@ -160,6 +160,16 @@ Enables a scroll method. Permitted values are
 Not all devices support all options, if an option is unsupported, the
 default scroll option for this device is used.
 .TP 7
+.BI "Option \*qScrollPixelDistance\*q \*q" int \*q
+Sets the movement distance, in "pixels", required to trigger one logical
+wheel click. This option only applies to the scroll methods
+.BI twofinger,
+.BI edge,
+.BI button.
+See section
+.B SCROLL PIXEL DISTANCE
+for more details.
+.TP 7
 .BI "Option \*qSendEventsMode\*q \*q" (disabled|enabled|disabled-on-external-mouse) \*q
 Sets the send events mode to disabled, enabled, or "disable when an external
 mouse is connected".
@@ -279,6 +289,10 @@ Indicates which scroll methods are available on this device.
 3 boolean values (8 bit, 0 or 1), in order "two-finger", "edge", "button".
 Indicates which scroll method is currently enabled on this device.
 .TP 7
+.BI "libinput Scroll Pixel Distance"
+1 32-bit value (nonzero, with additional implementation-defined range checks).
+Changes the movement distance required to trigger one logical wheel click.
+.TP 7
 .BI "libinput Send Events Modes Available"
 2 boolean values (8 bit, 0 or 1), in order "disabled" and
 "disabled-on-external-mouse". Indicates which send-event modes are available
@@ -396,6 +410,17 @@ it takes left-handed-ness into account.
 .TP
 This feature is provided by this driver, not by libinput.
 
+.SH SCROLL PIXEL DISTANCE
+The X server does not support per-pixel scrolling but it does support
+smooth scrolling. All scroll events however are based around a logical
+unit of scrolling (traditionally corresponding to a wheel click).
+It is thus not possible to scroll by 10 pixels, but it is possible for a
+driver to scroll by 1/10th of a logical wheel click.
+.PP
+libinput provides scroll data in pixels. The \fBScrollPixelDistance\fR
+option defines the amount of movement equivalent to one wheel click. For
+example, a value of 50 means the user has to move a finger by 50 pixels to
+generate one logical click event and each pixel is 1/50th of a wheel click.
 .SH BUGS
 This driver does not work with \fBOption \*qDevice\*q\fR set to an event
 node in \fI/dev/input/by-id\fR and \fI/dev/input/by-path\fR. This can be