Add tearing support (#1390)

Co-authored-by: Andrew J. Hesford <ajh@sideband.org>

docs/labwc-actions.5.scd

@@ -154,6 +154,9 @@ Actions are used in menus and keyboard/mouse bindings.
 	the usual keybinds will function again until switching back to the
 	original window. There can be multiple windows with this mode set.
 
+*<action name="ToggleTearing" />*
+	Toggles tearing for the focused window.
+
 *<action name="FocusOutput" output="HDMI-A-1" />*
 	Give focus to topmost window on given output and warp the cursor
 	to the center of the window. If the given output does not contain

docs/labwc-config.5.scd

@@ -109,6 +109,7 @@ this is for compatibility with Openbox.
   <decoration>server</decoration>
   <gap>0</gap>
   <adaptiveSync>no</adaptiveSync>
+  <allowTearing>no</allowTearing>
   <reuseOutputMode>no</reuseOutputMode>
 </core>
 ```
@@ -128,6 +129,12 @@ this is for compatibility with Openbox.
 	*fullscreen* enables adaptive sync whenever a window is in fullscreen
 	mode.
 
+*<core><allowTearing>* [yes|no]
+	Allow tearing to reduce input lag. Default is no.
+	This option requires setting the environment variable WLR_DRM_NO_ATOMIC=1.
+
+	*yes* allow tearing if requested by the active window.
+
 *<core><reuseOutputMode>* [yes|no]
 	Try to re-use the existing output mode (resolution / refresh rate).
 	This may prevent unnecessary screenblank delays when starting labwc

docs/rc.xml.all

@@ -11,6 +11,7 @@
     <decoration>server</decoration>
     <gap>0</gap>
     <adaptiveSync>no</adaptiveSync>
+    <allowTearing>no</allowTearing>
     <reuseOutputMode>no</reuseOutputMode>
   </core>