sdl: Add zoom hot keys

Allow to enlarge or shrink the screen via CTRL-ALT-+/-. In contrast to
scaling the window, these controls always preserve the aspect ratio of
the current console.

CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-doc.texi

@@ -288,6 +288,14 @@ then the modifier is Ctrl-Alt-Shift (instead of Ctrl-Alt) and if you use
 @kindex Ctrl-Alt-f
 Toggle full screen
 
+@item Ctrl-Alt-+
+@kindex Ctrl-Alt-+
+Enlarge the screen
+
+@item Ctrl-Alt--
+@kindex Ctrl-Alt--
+Shrink the screen
+
 @item Ctrl-Alt-u
 @kindex Ctrl-Alt-u
 Restore the screen's un-scaled dimensions

ui/sdl.c

@@ -651,6 +651,19 @@ static void sdl_refresh(DisplayState *ds)
                             absolute_mouse_grab();
                         }
                         break;
+                    case 0x1b: /* '+' */
+                    case 0x35: /* '-' */
+                        if (!gui_fullscreen) {
+                            int width = MAX(real_screen->w +
+                                            (keycode == 0x1b ? 50 : -50), 160);
+                            int height = (ds_get_height(ds) * width) /
+                                         ds_get_width(ds);
+
+                            sdl_scale(ds, width, height);
+                            vga_hw_invalidate();
+                            vga_hw_update();
+                            gui_keysym = 1;
+                        }
                     default:
                         break;
                     }