- Upstream now provides tarballs with source only as git tags
from github so update Source0 path.

- Add requires on various X extensions linked to opencv_ts module.
As those are present in the .pc file we need it anyway.

- Update to OpenCV 2.4.8
More info at:
http://opencv.org/opencv-2-4-8.html
The brief list of changes:
* NVidia CUDA support on Android devices with CUDA capable SoC and
CUDA sample;
* Concurrent kernel execution and user defined context support for
OpenCL;
* Integration with Intel Perceptual SDK and new depth sensors support
for Windows;
* 32 reported bugs have been closed;
* 139 pull requests have been merged;