plugins/elements/gstinputselector.c: Move the select-all logic into the activation...

plugins/elements/gstinputselector.c: Move the select-all logic into the activation of the currently selected pad. We want...

Original commit message from CVS:
* plugins/elements/gstinputselector.c: (gst_selector_pad_bufferalloc),
(gst_selector_pad_chain), (gst_input_selector_getcaps),
(gst_input_selector_activate_sinkpad):
Move the select-all logic into the activation of the currently selected
pad. We want to remember the last pad with activity in select-all mode.
Fix the getcaps function, we can produce the union of the upstream caps
in select-all mode, not the intersection like proxy_getcaps() does.