Create VideoCamera base class and move camera settings functions to it.

This makes them available for both UsbCamera and HttpCamera / AxisCamera.

To avoid virtual functions in the public-facing interface, move the
implementation of the camera settings functions to the core library.

src/CvSourceImpl.cpp

@@ -82,6 +82,41 @@ void CvSourceImpl::SetStringProperty(int property, llvm::StringRef value,
   UpdatePropertyValue(property, true, 0, value);
 }
 
+// These are only valid for cameras (should never get called)
+
+void CvSourceImpl::SetBrightness(int brightness, CS_Status* status) {
+  *status = CS_INVALID_HANDLE;
+}
+
+int CvSourceImpl::GetBrightness(CS_Status* status) const {
+  *status = CS_INVALID_HANDLE;
+  return 0;
+}
+
+void CvSourceImpl::SetWhiteBalanceAuto(CS_Status* status) {
+  *status = CS_INVALID_HANDLE;
+}
+
+void CvSourceImpl::SetWhiteBalanceHoldCurrent(CS_Status* status) {
+  *status = CS_INVALID_HANDLE;
+}
+
+void CvSourceImpl::SetWhiteBalanceManual(int value, CS_Status* status) {
+  *status = CS_INVALID_HANDLE;
+}
+
+void CvSourceImpl::SetExposureAuto(CS_Status* status) {
+  *status = CS_INVALID_HANDLE;
+}
+
+void CvSourceImpl::SetExposureHoldCurrent(CS_Status* status) {
+  *status = CS_INVALID_HANDLE;
+}
+
+void CvSourceImpl::SetExposureManual(int value, CS_Status* status) {
+  *status = CS_INVALID_HANDLE;
+}
+
 bool CvSourceImpl::SetVideoMode(const VideoMode& mode, CS_Status* status) {
   // can't set video mode on OpenCV source
   return false;