minor refactoring of gpu tests

#1713 Added the possibility of setting user_block_size manually for gpu::matchTemplate function (and gpu::convolve). Added a buffer param into these functions. Removed using of 2^n block sizes when it's not necessary.

added more tests for gpu cvtColor
fixed RGB->YUV conversion

fixed a few compile problems in SparseMat code; fixed iteration over an empty sparse matrix (ticket #1587)

disabled NVidia Visualization test, it's functionality (draw rectangles) doesn't used in gpu module