gl/egl/display: Add support EGLDevice display type
Simple addition for supporting EXT_platform_device typed display. It's a kind of special display type (part of EGL specification) which has no window at all. To use EGLDevice explicitly, set environment "GST_GL_WINDOW=egl-device"
See also https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_platform_device.txt
Edited by Seungha Yang