libcamera: Default to auto-focus & auto-exposure

libcamera says that cameras should default to manual focus mode.  This
means that unless pipewire clients specifically change this control,
users with an autofocus-capable camera are left with an out-of-focus
image.  This patch sets the autofocus mode to continuous and enables
auto-exposure (as the default for this is unspecified).

Testing with an imx708 on Raspberry Pi OS on a Raspberry Pi 4, before
this patch the image was generally out of focus in Firefox/webrtc, after
this patch autofocus works correctly.

spa/plugins/libcamera/libcamera-source.cpp

@@ -163,6 +163,7 @@ struct impl {
 	struct spa_source source = {};
 
 	ControlList ctrls;
+	ControlList initial_controls;
 	bool active = false;
 	bool acquired = false;