Fix crashes when opening or closing readers and exporting raw video

[jonoomph]

This fixes two libopenshot stability issues:

• Overlapping reader Open() and Close() calls now check whether the reader is open while holding the existing mutex, preventing duplicate initialization or cleanup.
• Raw-video export now gives each packet its own buffer, preventing a double-free. It also writes all image planes and correctly scales packet timestamps.

Regression tests cover overlapping reader calls, reopening and reading afterward, and raw-video packet data, timestamps, and decoded colors. A dedicated Linux CI step checks these cases with Valgrind.

[jonoomph]

Fix crashes when opening or closing readers and exporting raw video