Map depth and stencil buffers activation from desktop APIs to WebGL

Following the recent pull request (merged) from wsmind, I added the support for enabling/disabling the use
of depth and stencil buffer in the GLUT, SDL and GLFW libraries (EGL is missing as the context attributes are currently not parsed in library_egl.js).

I also added tests to verify that the requested context attributes values (antialiasing included) are correctly taken into account (wether they are activated or not). Those tests first check that the WebGL implementation supports the requested attributes.