Camera is currently stopped and started in the spa_libcamera_set_format()
function, but this leads to a segfault due attempting to access a buffers
field of an already freed libcamera::FrameBufferAllocator instance.
The FrameBufferAllocator instance is freed in LibCamera::stop(), that is
called by spa_libcamera_stream_off() as handler of the node commands
SPA_NODE_COMMAND_Pause and SPA_NODE_COMMAND_Suspend.
Since the camera was already stopped, there's no need to attempt to stop
it again. In fact, the camera shouldn't be stopped/started at all in the
spa_libcamera_set_format() function but instead only as an action of the
SPA_NODE_COMMAND_{Pause,Suspend} and SPA_NODE_COMMAND_Start commands.
And same for the stop that's done in the LibCamera::close() function, it
shouldn't be needed because the camera is already stopped before closing.
Fixes#1513
There is no need to set and validate the configuration when the camera
is started, since this was already made when the camera was opened.
The configuration only needs to be set again if the port format changes
due a SPA_PARAM_Format parameter.
SPA_MEMBER is misleading, all we're doing here is pointer+offset and a
type-casting the result. Rename to SPA_PTROFF which is more expressive (and
has the same number of characters so we don't need to re-indent).
