vrend: Add get_egl_display callback

This is required for support of untyped resources on virtio devices which create their own egl context.

Currently untyped resources support is reported to mesa only in case virglrenderer creates it's own egl context from scratch. The problem that this context is not "sharable" with device context and if it's created vrend uses it instead of context callbacks implemented by device. Thus such approach is not suitable for devices with it's own egl context.

The only thing that is required for untyped resources support is initialized eglDisplay handle that is required for eglCreateImageKHR, thus add callback for device to share it.

Untyped resources feature support is required for sharing Vulkan resources with OpenGL stack.

/cc @olv