appsink: add missing make_writable call

There may be the case, which I managed to trigger with a flushing seek in a pipeline ending with an appsink, when the affected branch of code in _render is executed. gst_sample_set_caps requires the sample to be writable, which made this function fail before this change.

This MR adds gst_sample_make_writable call before gst_sample_set_caps, similarly to how sample writability is assured in the rest of appsink code