vout: don't clock_Convert forced pictures

This MR is a prerequisite for !5176

In that future MR, all clocks will be started from vlc_clock_Start manually (from input/decoder.c), thus vlc_clock_ConvertToSystem won't set up the monotonic clock when first called (it will be done by the new Start function).

In order to keep the fact that vlc_clock_ConvertToSystem should always return a valid TS (because we don't want to handle possible failure from every call), vlc_clock_ConvertToSystem need be called after vlc_clock_Start (and before vlc_clock_Reset).

Therefore, vlc_clock_ConvertToSystem should not be called from the vout when handling a forced picture. Indeed, a first and forced picture is always sent to the vout when the clock is not yet started (it will be Started after the buffering is done). This MR will prevent to call clock convert function in such case.