doc: Fix the InputCapture barrier examples

A barrier is either at the top or at the left edge, not both. This means
the actual barriers need to go from zone->x to zone->width - 1, the next
pixel (zone->x + zone->width) is the first pixel of the adjacent zone if
any.

We could allow this in the spec but it makes implementations a lot
harder: on a two-screen setup the edge pixel would need special casing,
the barrier 0-1920 and 1920-3840 are both valid on either screen despite
having a single-pixel overlap. An easier approach is to make sure each
barrier only covers the pixels intended, i.e. 0-1919 and 1920 to 3839 in
this case.

data/org.freedesktop.portal.InputCapture.xml

@@ -205,21 +205,24 @@
         (from compositor-determined input devices) are sent to the application
         via the transport layer.
 
-        Pointer barriers are situated on the top or left of their respective pixels and a
-        pointer barrier must be situated at the outside boundary of the union of all zones.
+        Pointer barriers are situated on the top (for horizontal barriers) or left
+        (for vertical barriers) edge of their respective pixels and their width or height
+        is inclusive each pixel's width or height. In other words, a barrier spanning
+        x1=0, x2=1 is exactly two pixels wide. A pointer barrier must be situated at the
+        outside boundary of the union of all zones.
         A pointer barrier must be fully contained within one zone.
 
         For example, consider two zones of size 1920x1080 with offsets
         0,0 and 1920,0, respectively. (i.e. a left-right dual-monitor setup).
         The following pointer barriers are permitted:
 
         <itemizedlist>
-        <listitem><para>top edge of left screen: <literal>x1=0,y1=0,x2=1920,y1=0</literal></para></listitem>
-        <listitem><para>bottom edge of left screen: <literal>x1=0,y1=1080,x2=1920,y1=1080</literal></para></listitem>
-        <listitem><para>top edge of right screen: <literal>x1=1920,y1=0,x2=3840,y1=0</literal></para></listitem>
-        <listitem><para>bottom edge of right screen: <literal>x1=1920,y1=1080,x2=3840,y1=1080</literal></para></listitem>
-        <listitem><para>left edge of left screen: <literal>x1=0,y1=0,x2=0,y1=1080</literal></para></listitem>
-        <listitem><para>right edge of right screen: <literal>x1=3840,y1=0,x2=3840,y1=1080</literal></para></listitem>
+        <listitem><para>top edge of left screen: <literal>x1=0,y1=0,x2=1919,y1=0</literal></para></listitem>
+        <listitem><para>bottom edge of left screen: <literal>x1=0,y1=1080,x2=1919,y1=1080</literal></para></listitem>
+        <listitem><para>top edge of right screen: <literal>x1=1920,y1=0,x2=3839,y1=0</literal></para></listitem>
+        <listitem><para>bottom edge of right screen: <literal>x1=1920,y1=1080,x2=3839,y1=1080</literal></para></listitem>
+        <listitem><para>left edge of left screen: <literal>x1=0,y1=0,x2=0,y1=1079</literal></para></listitem>
+        <listitem><para>right edge of right screen: <literal>x1=3840,y1=0,x2=3840,y1=1079</literal></para></listitem>
         </itemizedlist>
 
         A pointer barrier is considered triggered when the pointer would