opencv_trail_effect.cpp: fix showing the background

When showing the background OpenCV would fail with this message:

  OpenCV Error: Assertion failed (channels() == CV_MAT_CN(dtype)) in copyTo, file /build/opencv-lY8WRY/opencv-3.2.0+dfsg/modules/core/src/copy.cpp, line 259
  terminate called after throwing an instance of 'cv::Exception'
    what():  /build/opencv-lY8WRY/opencv-3.2.0+dfsg/modules/core/src/copy.cpp:259: error: (-215) channels() == CV_MAT_CN(dtype) in function copyTo

Initializing the background to the same format of the input frame seems
to fix this.