avfilter/deinterlace_vaapi: keep previous PTS in 64 bits

prev_pts stores an AVFrame timestamp but is declared as int. When the
input PTS exceeds the range of int, the narrowing conversion changes
the value used to extrapolate the second field timestamp at EOF.

With a 1/90000 input time base and 32-bit int, this can happen after
about 6 hours and 38 minutes. On the tested platform, the final output
timestamp jumps forward by 2^32 ticks in the 1/180000 output time base,
or about 23861 seconds. This can greatly inflate the output duration.

Store prev_pts as int64_t to preserve the input timestamp. The EOF
calculation and the normal field interpolation remain unchanged.

Signed-off-by: yusuke iijima <yiijima@gmail.com>

libavfilter/vf_deinterlace_vaapi.c

@@ -47,7 +47,7 @@ typedef struct DeintVAAPIContext {
     int                extra_delay_for_timestamps;
 
     int                eof;
-    int                prev_pts;
+    int64_t            prev_pts;
 } DeintVAAPIContext;
 
 static const char *deint_vaapi_mode_name(int mode)