gtkgstsink.c: Support EGL on Windows as well

gtkgstsink.c: Support EGL on Windows as well

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.