input.conf: add default bindings for changing window scale

These are very much inspired by the hardcoded Cocoa bindings on OSX.

Fixes #2500.

DOCS/man/mpv.rst

@@ -192,23 +192,19 @@ corresponding adjustment, or the software equalizer (``--vf=eq``).)
 7 and 8
     Adjust saturation.
 
-(The following keys are valid only on OS X.)
-
-command + 0
+Alt+0 (and command+0 on OSX)
     Resize video window to half its original size.
-    (On other platforms, you can bind keys to change the ``window-scale``
-    property.)
 
-command + 1
+Alt+1 (and command+1 on OSX)
     Resize video window to its original size.
 
-command + 2
+Alt+2 (and command+2 on OSX)
     Resize video window to double its original size.
 
-command + f
+command + f (OSX only)
     Toggle fullscreen (see also ``--fs``).
 
-command + [ and command + ]
+command + [ and command + ] (OSX only)
     Set video window alpha.
 
 (The following keys are valid if you have a keyboard with multimedia keys.)

etc/input.conf

@@ -100,6 +100,9 @@
 #6 add gamma 1
 #7 add saturation -1
 #8 add saturation 1
+#Alt+0 set window-scale 0.5
+#Alt+1 set window-scale 1.0
+#Alt+2 set window-scale 2.0
 # toggle deinterlacer (automatically inserts or removes required filter)
 #d cycle deinterlace
 #r add sub-pos -1                       # move subtitles up