This commit makes the loading of the GModules threadsafe, and
always first tries to load the symbol for the GL library that
is selected for the current context. Only then it falls back
to looking into the current module (NULL), and only as a last
resort the context specific function (e.g. eglGetProcAddress())
is called.
Also add configure parameters to select the names of the library
modules instead of using the defaults, and let the defaults be
independent of the G_MODULE_SUFFIX.
https://bugzilla.gnome.org/show_bug.cgi?id=728753
GstGLDisplayX11 holds the display connection and name. Each thread requires
it's own X11 Display connection (initialised from name) due to the fact that
we do not want to call XInitThreads(). Doing so would result in segfaults
when integrating with GUI toolkits Gtk, Qt, etc.
The Display connection is for OpenGL platforms where a constant display is
required in order to share contexts (egl). In the case of a wrapped context
(added later), we do not have GstGLWindow to retreive the display from so a
'master' connection is used instead.
