lavf: make overlay_qsv work based on framesync

The existing version which was cherry-picked from Libav does not work
with FFmpeg framework, because ff_request_frame() was totally
different between Libav (recursive) and FFmpeg (non-recursive).
The existing overlay_qsv implementation depends on the recursive version
of ff_request_frame to trigger immediate call to request_frame() on input pad.
But this has been removed in FFmpeg since "lavfi: make request_frame() non-recursive."
Now that we have handy framesync support in FFmpeg, so I make it work
based on framesync. Some other fixing which is also needed to make
overlay_qsv work are put in a separate patch.
Signed-off-by: Ruiling Song <ruiling.song@intel.com>

libavfilter/Makefile

@@ -270,7 +270,7 @@ OBJS-$(CONFIG_OSCILLOSCOPE_FILTER)           += vf_datascope.o
 OBJS-$(CONFIG_OVERLAY_FILTER)                += vf_overlay.o framesync.o
 OBJS-$(CONFIG_OVERLAY_OPENCL_FILTER)         += vf_overlay_opencl.o opencl.o \
                                                 opencl/overlay.o framesync.o
-OBJS-$(CONFIG_OVERLAY_QSV_FILTER)            += vf_overlay_qsv.o
+OBJS-$(CONFIG_OVERLAY_QSV_FILTER)            += vf_overlay_qsv.o framesync.o
 OBJS-$(CONFIG_OWDENOISE_FILTER)              += vf_owdenoise.o
 OBJS-$(CONFIG_PAD_FILTER)                    += vf_pad.o
 OBJS-$(CONFIG_PALETTEGEN_FILTER)             += vf_palettegen.o