glcontext: add support for influencing the backing configuration

glcontext: add support for influencing the backing configuration

New API:
- gst_gl_context_get_config()
- gst_gl_context_request_config()

A GL context configuration is a GstStructure that has some well-known
names for common values that can also be extended in platform-specific
ways if necessary.

Wrapped OpenGL contexts may be able to retrieve the GL context
configuration depending on the platform.  If that information is
available, GstGLContext will attempt to create an context that matches
the shared OpenGL context config unless gst_gl_context_request_config()
has been called.

A new environment variable 'GST_GL_CONFIG' will be read to influence the
configuration chosen.  The environment variable will only be used as a
fallback if gst_gl_context_request_config() has not been called.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/809>