mpv: Define maximum window size

This makes it so that mpv won't take up the entire screen when executed,
assuming it's dealing with a video resolution equal to or greater than
the current display.

mpv/.config/mpv/mpv.conf

@@ -47,3 +47,6 @@ sub-blur=10
 
 # Start in fullscreen by default
 fullscreen=yes
+
+# Don't allow new windows to have a size larger than 90% of the screen size
+autofit-larger=90%x90%