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
89392b2a6d