Add --render-fit feature

Add an option to configure how the rendering fits the window.

The default, `--render-fit=letterbox`, preserves the aspect ratio
and fits the window as best as possible, adding black bars at
the top/bottom or left/right if needed. This has been the only behavior
scrcpy supported so far.

Another mode, `--render-fit=disabled`, renders the display at the
top-left corner without scaling. This mode will be useful for virtual
display resizing.

app/data/bash-completion/scrcpy

@@ -80,6 +80,7 @@ _scrcpy() {
         --record-format=
         --record-orientation=
         --render-driver=
+        --render-fit=
         --require-audio
         -s --serial=
         -S --turn-screen-off
@@ -176,6 +177,10 @@ _scrcpy() {
             COMPREPLY=($(compgen -W 'direct3d opengl opengles2 opengles metal software' -- "$cur"))
             return
             ;;
+        --render-fit)
+            COMPREPLY=($(compgen -W 'letterbox disabled' -- "$cur"))
+            return
+            ;;
         --shortcut-mod)
             # Only auto-complete a single key
             COMPREPLY=($(compgen -W 'lctrl rctrl lalt ralt lsuper rsuper' -- "$cur"))