Allow to unscale the output window with a Ctrl-Alt-u hotkey

Signed-off-by: Nmalc <av1474@comtv.ru>

qemu-doc.texi

@@ -241,6 +241,9 @@ During the graphical emulation, you can use the following keys:
 @item Ctrl-Alt-f
 Toggle full screen
 
+@item Ctrl-Alt-u
+Restore the screen's un-scaled dimensions
+
 @item Ctrl-Alt-n
 Switch to virtual console 'n'. Standard console mappings are:
 @table @emph

sdl.c

@@ -569,6 +569,12 @@ static void sdl_refresh(DisplayState *ds)
                         toggle_full_screen(ds);
                         gui_keysym = 1;
                         break;
+                    case 0x16: /* 'u' key on US keyboard */
+                        scaling_active = 0;
+                        sdl_resize(ds);
+                        vga_hw_invalidate();
+                        vga_hw_update();
+                        break;
                     case 0x02 ... 0x0a: /* '1' to '9' keys */
                         /* Reset the modifiers sent to the current console */
                         reset_keys();