Don't pass in the driver context to `camera_driver::device_list_new`

- Unlike some other drivers, the camera driver is only initialized when a core actually asks for it
- Therefore, the driver context isn't available when it's time to ask for devices
- I could refactor the camera driver to be created earlier, but then I'd need to refactor the other drivers as well
- That'll come later in another PR

camera/camera_driver.c

@@ -34,8 +34,8 @@ static bool nullcamera_start(void *data) { return true; }
 static bool nullcamera_poll(void *a,
       retro_camera_frame_raw_framebuffer_t b,
       retro_camera_frame_opengl_texture_t c) { return true; }
-static struct string_list *nullcamera_device_list_new(const void *data) { return NULL; }
-static void nullcamera_device_list_free(const void *data, struct string_list *devices) {}
+static struct string_list *nullcamera_device_list_new() { return NULL; }
+static void nullcamera_device_list_free(struct string_list *devices) {}
 
 static camera_driver_t camera_null = {
    nullcamera_init,

camera/camera_driver.h

@@ -49,8 +49,8 @@ typedef struct camera_driver
          retro_camera_frame_raw_framebuffer_t frame_raw_cb,
          retro_camera_frame_opengl_texture_t frame_gl_cb);
 
-   struct string_list *(*device_list_new)(const void *driver_context);
-   void (*device_list_free)(const void *driver_context, struct string_list *devices);
+   struct string_list *(*device_list_new)(void);
+   void (*device_list_free)(struct string_list *devices);
 
    const char *ident;
 } camera_driver_t;

camera/drivers/ffmpeg.c

@@ -500,7 +500,7 @@ static bool ffmpeg_camera_poll(
    return true;
 }
 
-static struct string_list *ffmpeg_camera_device_list_new(const void *driver_context)
+static struct string_list *ffmpeg_camera_device_list_new(void)
 {
    const AVInputFormat *input = NULL;
    struct string_list *list = string_list_new();
@@ -517,7 +517,7 @@ static struct string_list *ffmpeg_camera_device_list_new(const void *driver_cont
    return list;
 }
 
-static void ffmpeg_camera_device_list_free(const void *driver_context, struct string_list *devices)
+static void ffmpeg_camera_device_list_free(struct string_list *devices)
 {
    struct string_list *sl = (struct string_list*)devices;