Fix realsense viewer on macos

[devernay]

macOS does not provide an OpenGL 3.2 compatibility profile: only OpenGL 2.1 is available.
Because of the shading langage version used (1.10), we cannot use a core profile either.
The solution is to check whether the required functionality is available, and provide fallback replacements from extensions.
There is also a clang-specific test to let the unit tests compile.

[SHTseng]

UI tools such realsense viewer, depth quality tool in macos get segmentation fault on the latest release. They are both work with this pull request. I tested on a 2017 mbp with macos 10.13.6, and connect the d435i to the computer via usb-c to usb-c wire.

[dorodnic]

Hi @devernay
Thanks for the fix, that's my bad. I was really struggling with this problem and will test the fix ASAP.
I'm trying to upstream GLSL rendering and calculation of point-cloud and other CPU consuming parts for Viewer and general use. Due to the Mac OS problem, I tried to roll-back all usages of GLSL for last release, but must have missed one.

[dorodnic]

Worked like a charm 👍 I completely missed that glad script can generate extensions in this way.
Also, tested this PR on Ubuntu and Windows 10.

I'm not sure why emplace_back was giving trouble, AFAIK it should compile with most recent versions of clang, and we do have clang 9.1 in Travis. But push_back is equally good.

[dorodnic]

One more thing - I retargeted the PR to the development branch, as it is our staging area prior to release.
(see CONTRIBUTING.md)
Obviously, we will work to get master back to stable state ASAP, but it will be easier for us if everything is merged to dev first.