Add different image formats

Java side:
```java
class FakeCam {
public static final int V4L2_PIX_FMT_YUV420 = 1;
public static final int V4L2_PIX_FMT_RGB24 = 2;
}
```

C side:
```c
#define FMT_YUV420 1
#define FMT_RGB24 2

JNIEXPORT jint JNICALL Java_com_harium_hci_fakecam_FakeCam_open(JNIEnv* env, jobject obj, jstring device, jint width, jint height, jint format) {
switch(format) {
case V4L2_PIX_FMT_YUV420:
format = V4L2_PIX_FMT_YUV420;
break;
....
}
initWithFormat(..., format);
}
```