Move OpenCL SURF to nonfree module

Fixed wrong index in prefilterXSobel function

added test for bug #1448 and hopefully fixes the bug #2898

GPU test fixes

add hack to disable optimization of linear svms; improved precision of optimize_linear_svm; add the relevant test, which however requires some big database (so it's disabled by default)

1. Added more sync to reduction.
2. Turned off Image2D feature. Probably its support is not detected correctly.
3. Temporary disabled descriptor tests - can't localize a problem of the ocl descriptor.