staging support

This tree is able to take pictures using staging drivers.

It needs libcamera from https://source.puri.sm/dorota.czaplejewicz/libcamera/-/commit/86dbed4ae932be295096d3774a4c71f45f5e558a. To use a local install:

1. Build libcamera with --prefix=/home/purism/caminstall
2. Build megapixels with PKG_CONFIG_PATH=/home/purism/caminstall/lib/aarch64-linux-gnu/pkgconfig/ meson ../megapixels/ --prefix=/home/purism/mpi
3. Run megapixels with LD_LIBRARY_PATH=/home/purism/caminstall/lib/aarch64-linux-gnu/ ./mpi/bin/megapixels

What works:

• preview on both cameras
• photos on both cameras

What does not work:

• not Evergreen (needs paths filled in)
• not raw RGB (mode conversion table needs filled in)