The filter supports two inputs and (implicitly) scaling the second input
during composition, unlike the software overlay.

The code has been separated into common interface and qsv overlay
implementation. The common part mainly creates the qsv session and
manages the surface which is nearly the same for all qsv filters.
So the qsvvpp.c/qsvvpp.h API can be used by other QSV vpp filters
to reduce code redundancy.

Usage:
 -hwaccel qsv -c:v mpeg2_qsv -r 25 -i in.m2v -hwaccel qsv -c:v h264_qsv
 -i in.h264 -filter_complex
 "overlay_qsv=eof_action=repeat:x=(W-w)/2:y=(H-h)/2"  -b 2M -maxrate 3M
 -c:v h264_qsv -y out.h264

Two inputs should have different sizes otherwise one will be completely
covered or you need to scale the second input as follows:
  -hwaccel qsv -c:v mpeg2_qsv -r 25 -i in.m2v -hwaccel qsv -c:v h264_qsv
  -i in.h264 -filter_complex
  "overlay_qsv=w=720:h=576:x=(W-w)/2:y=(H-h)/2" -b 2M -maxrate 3M -c:v
  h264_qsv -y out.h264
Signed-off-by: ChaoX A Liu <chaox.a.liu@gmail.com>
Signed-off-by: Zhengxu Huang <zhengxu.maxwell@gmail.com>
Signed-off-by: Andrew Zhang <huazh407@gmail.com>
  Change-Id: I5c381febb0af6e2f9622c54ba00490ab99d48297
Signed-off-by: Maxym Dmytrychenko <maxim.d33@gmail.com>