libcamera: framebuffer: Allocate metadata planes at construction time

The metadata planes are allocated by V4L2VideoDevice when dequeuing a
buffer. This causes the metadata planes to only be allocated after a
buffer gets dequeued, and doesn't provide any strong guarantee that
their number matches the number of FrameBuffer planes. The lack of this
invariant makes the FrameBuffer class fragile.

As a first step towards fixing this, allocate the metadata planes when
the FrameBuffer is constructed. The FrameMetadata API should be further
improved by preventing a change in the number of planes.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>

src/libcamera/framebuffer.cpp

@@ -210,6 +210,8 @@ FrameBuffer::FrameBuffer(const std::vector<Plane> &planes, unsigned int cookie)
 	: Extensible(std::make_unique<Private>()), planes_(planes),
 	  cookie_(cookie)
 {
+	metadata_.planes.resize(planes_.size());
+
 	unsigned int offset = 0;
 	bool isContiguous = true;
 	ino_t inode = 0;