Internally libyuv uses malloc & free for short time dynamic memory
allocation during image transformation. The converted images are
such large, that the Libc allocator will create and destroy new Genode
dataspace per image. In time sensitive code paths, the overhead can be
noticeable by the caller of the image transformation.
The patch adds the option to register callbacks in the libyuv library to
implement the image allocation by users of the library. They may implement
caching strategies to avoid the overhead, e.g. as seen with qemu-usb and
the webcam model.
If `select_from_ports` is evaluated lazily, we might miss a port during
the dependency check. A way to prevent this is to use the `:=` operator.
Fixesgenodelabs/genode#4618
