Add support to look for and use the EGL context in Windows if it was activated
instead of desktop OpenGL.
GstGL may have been built with or without EGL/libANGLE support, so if it were,
check in GstGL whether we have gst_gl_display_new_with_type() to create a
GstGLDisplay that is of the GST_GL_WINDOW_WIN32 type when we are using
Desktop OpenGL (WGL), otherwise we show messages indicating that envvars
need to be set to initialize GstGL properly.
Due to a bug in GstGL, the GstGLContext can only be set up successfully
if one of the following is true:
* An OpenGL 3.x or later emulator, such as Mesa is used (for WGL)
* The latest GstGL master is being used, at the time of writing (for
WGL)
* GTK, libepoxy and GstGL are all built only with WGL support (for WGL)
* EGL is being used in GTK at runtime
Special thanks to Matthew Waters for the help during the process.
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