QDmaBufVideoBuffer: use QVarLengthArray


the `m_planeCount(std::clamp(...)` had the wrong signature. avoiding the
need for a separate m_planeCount by using a `QVarLengthArray`.

As dive-by use qt_save_close to catch EINTR during close

Pick-to: 6.12
Change-Id: I9a6fcfbc0e26ad4da172728573e2a4f8c8dd7207
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
commit6611bbfafbfbb4c52680e2775acfc7ee81ebf0e6
parent61e6891e4c199ae2cec7bd22edc153007af59494
authorTim Blechmann <tim.blechmann@qt.io>
committerTim Blechmann <tim.blechmann@qt.io>
commit timeTue, 20 Oct 2026 12:38:03 +0000