input.conf: bind Ctrl+left click to drag-to-pan

An alternative is to bind to right click only for images, but since we
will likely bind right click to the context menu once it works on
platforms other than Windows, bind to ctrl+left click to not have to
change it later.

DOCS/man/mpv.rst

@@ -373,6 +373,10 @@ in the mpv git repository.
 Mouse Control
 -------------
 
+Ctrl+left click
+    Pan while holding the button, keeping the clicked part of the video under
+    the cursor.
+
 Left double click
     Toggle fullscreen on/off.
 

etc/input.conf

@@ -33,6 +33,7 @@
 #MBTN_RIGHT    cycle pause         # toggle pause/playback mode
 #MBTN_BACK     playlist-prev       # skip to the previous file
 #MBTN_FORWARD  playlist-next       # skip to the next file
+#Ctrl+MBTN_LEFT script-binding positioning/drag-to-pan # pan around the clicked point
 
 # Mouse wheels, touchpad or other input devices that have axes
 # if the input devices supports precise scrolling it will also scale the