GStreamer: derive DMABuf video buffers from QDmaBufVideoBuffer

Replace the bespoke EGL import in QGstVideoBuffer with a new
QGstDmaBufVideoBuffer subclass that converts the GstBuffer into
plain DMABuf planes and reuses the shared importer and CPU mapping.
QGstVideoBuffer is left with CPU and GLTexture memory only, and a
new QGstBufferProvider interface exposes the GstBuffer.
Unsupported buffers fall back to the CPU path as before.

Pick-to: 6.12
Change-Id: I5797bfd6a2c0ac065d810d476e8d055801a1dd23
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
Reviewed-by: Lars Sutterud <lars.sutterud@qt.io>