rewrite & change convertFromGLBuffer() & convertToGLBuffer() into acquireGLBuffer() & releaseGLBuffer(), respectively

opengl sample: added buffer support

tested and fixed buffer support on Windows

change glFlush() call to glFinish()

added UMat::release() call; fixed functions' names

adopted & implemented API suggestion(s) from Alexander

fixed unreachable code warning

added more info to the mapGLBuffer/unmapGLBuffer description

Fixed errors in parallel_for based on pthreads

Fixing compiler errore & removing whitespaces

Fixing prallel_for_pthreads error and warnings on win

add template specialization Mat::push_back() for MatExpr paramters

extend push_back MatExpr to mat in unit test

cast to object instead of reference

test with multi-row MatExpr input

Signed-off-by: Maxim Kostin <v-maxkos@microsoft.com>

Signed-off-by: Maxim Kostin <v-maxkos@microsoft.com>

Conflicts:
	samples/data/detect_blob.png

TODO: Make nosse ver faster than Sobel
      Make sse ver faster than Sobel for BORDER_REPLICATE