Fix MKV hw encode issue (#150)

* Fix MKV hw encode issue

* Set global header using C++ API instead of C

Author: btmxh

example/api2-samples/api2-hw-encode.cpp

@@ -3,8 +3,6 @@
  *
  * Thanks to https://github.com/mojie126, for more-less complete sample.
  *
- * Known issues: error on the Matroska header writing.
- *
  */
 
 
@@ -155,6 +153,11 @@ int main(int argc, char **argv) {
     encoder.setTimeBase(Rational{1, 1000});
     encoder.setBitRate(vdec.bitRate());
 
+    // MKV format wants global header, so
+    // this fixes the issue with MKV container
+    if (ofrmt.flags() & AVFMT_GLOBALHEADER)
+        encoder.addFlags(AV_CODEC_FLAG_GLOBAL_HEADER);
+
     encoder.open(Codec(), ec);
     if (ec) {
         cerr << "Can't opent encodec\n";