Set the EGL_PLATFORM to offscreen when using hybris.

The offscreen rendering has been moved to an egl-platform hybris plugin, so the EGL_PLATFORM needs to be set correctly.

Nota Bene: this PR belongs to the following set of PR for webOS-ports: libwebos-gui, qt-webos-plugin, webappmanager, luna-sysmgr and meta-webos-ports.