correct error handling in gstgldownload.c
display: add display type enum
x11: add display subclass

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.
context: add support for wrapping external contexts
Merge branch 'master' into ci