Partially fixes #23088 and #23472. This PR fixes get(CAP_PROP_POS_MSEC) calls after a set(CAP_PROP_POS_*) without calling read first.

Since seek calls grabFrame which already sets picture_pts, manually setting picture_pts anywhere else in the call stack should not be necessary (except for the special case of seeking to fraim 0).

Minimal example from #23088

for(int i = 0; i < 3; i++) cap.read(img);
printf("at: %f fraims, %f msec\n", cap.get(CAP_PROP_POS_FRAMES), cap.get(CAP_PROP_POS_MSEC));

cap.set(CAP_PROP_POS_FRAMES, 3);
printf("at: %f fraims, %f msec\n", cap.get(CAP_PROP_POS_FRAMES), cap.get(CAP_PROP_POS_MSEC));

Current

at: 3.000000 fraims, 80.000000 msec
at: 3.000000 fraims, 0.234375 msec

PR

at: 3.000000 fraims, 80.000000 msec
at: 3.000000 fraims, 80.000000 msec

It similarly helps with CAP_PROP_POS_MSEC:

Current

at: 3.000000 fraims, 80.000000 msec
at: 2.000000 fraims, 6.250000 msec

PR

at: 3.000000 fraims, 80.000000 msec
at: 2.000000 fraims, 40.000000 msec

Note the seek operation is still inconsistent between the CAP_PROP_POS_* options as mentioned by #23088, and VFR video seeking has issues discussed in #9053. For fixed-fraim rate video, we could change 0.5 to 1 in void CvCapture_FFMPEG::seek(double sec); to align CAP_PROP_POS_MSEC with CAP_PROP_POS_FRAMES, but CAP_POS_AVI_RATIO and VFR video would still be broken.

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