gl: Add `gst_gl_context_new_with_type()` API
Currently the QML and GTK plugins (at least) assume that gst_gl_context_new()
creates a GL context that can share with the application/toolkit context. This usually works, but if e.g. GTK uses a GLX context but gst_gl_context_new()
creates an EGL context then this won't work because "Cannot share context with non-EGL context".
CC @ystreet