input.conf: add default bindings for changing window scale

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

Fixes #2500.

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