qt(6)/material: ensure that we always update the context in setBuffer()

qt(6)/material: ensure that we always update the context in setBuffer()

Scenario is that there are two (or more) GstGLContext's wrapping Qt's GL
context from either multiple qml(6)glsink or qml(6)glsrc elements.  Call flow is this:

1. material 1 setBuffer()
2. material 1 bind()
3. material 2 setBuffer()
4. material 2 bind()

If the call to setBuffer() reuses the same buffer as previous call, then the
qt context is not updated in the material.  If however the previously used qt
context by the material had been deactivated or freed, then bind() would fail
and could result in a critical like so:

gst_gl_context_thread_add: assertion 'context->priv->active_thread == g_thread_self ()' failed

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7978>