sunshine: size the virtual output to the client

The work laptop drives two 3840x2160 panels, so a fixed 1920x1080 capture
was being upscaled 2x on the client — fine for Big Picture's large UI,
visibly soft for terminal text on the Desktop app.

A headless wlroots output has no physical size, so it can take any mode.
Both apps now set HEADLESS-1 to SUNSHINE_CLIENT_WIDTH/HEIGHT/FPS in their
prep-cmd and restore 1920x1080 on undo. SWAYSOCK joins the environment
import so Sunshine's prep-cmds can reach the compositor at all.

hosts/mainframe/sunshine/apps.json

@@ -5,19 +5,25 @@
   "apps": [
     {
       "name": "Desktop",
-      "image-path": "desktop.png"
+      "image-path": "desktop.png",
+      "prep-cmd": [
+        {
+          "do": "swaymsg output HEADLESS-1 mode ${SUNSHINE_CLIENT_WIDTH}x${SUNSHINE_CLIENT_HEIGHT}@${SUNSHINE_CLIENT_FPS}Hz",
+          "undo": "swaymsg output HEADLESS-1 mode 1920x1080@60Hz"
+        }
+      ]
     },
     {
       "name": "Steam Big Picture",
       "image-path": "steam.png",
-      "detached": [
-        "setsid steam -gamepadui"
-      ],
       "prep-cmd": [
         {
-          "do": "",
-          "undo": "setsid steam -shutdown"
+          "do": "swaymsg output HEADLESS-1 mode ${SUNSHINE_CLIENT_WIDTH}x${SUNSHINE_CLIENT_HEIGHT}@${SUNSHINE_CLIENT_FPS}Hz",
+          "undo": "swaymsg output HEADLESS-1 mode 1920x1080@60Hz"
         }
+      ],
+      "detached": [
+        "setsid steam -gamepadui"
       ]
     }
   ]