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

changed output from fps to time (in msec). Notes for dx10 and dx11: the first measure is wrong (zero), the followiing are correct; measured time includes output text to surface.

pos should never exceed strlen, else the overflow check 'if (!len)' will wrongfully pass

1. Remove uncessary index calculation.
2. Use mad/mad24 as possible.
Signed-off-by: Yan Wang <yan.wang@linux.intel.com>

by this new function we can set libjpeg "scale_denom" parameter and load jpeg images scaled 1/2 - 1/4 - 1/8
other image formats resized after loading

compute fps changed (now only interop counts). fixed d3d10 and d3d11 print info method (there were issues with texture mapping)

Fixed polarity of ImageRegistration mode in setDepthGeneratorProperty function of CvCapture_OpenNI2 class

simplified sample's interface (remove odd "no processing" branch, add print to screen for processing mode)

it provides more expectable results to end-users.
based on feedback by Alexander Alekhin

a separate function instead of an overload not to change the ABI.

rename VideoCapture paramter 'device' to 'index' in CPP to reflect that
it allows specifying the API.
update comments to explain how to specify the API.