Draft: testing/rpicam-apps: new aport

Old raspicam/raspivid/raspistill API is deprecated. The new recommended API is libcamera-based. This MR adds rpicam-apps, which are replacements for raspivid/raspistill.

These are simple applications, built on top of libcamera, and are designed to emulate the function of the legacy stack built on Broadcom’s proprietary GPU code (some users will recognise these legacy applications as raspistill and raspivid).

https://www.raspberrypi.com/documentation/computers/camera_software.html#introduction

Unfortunately rpicam-apps (for now) needs a custom raspberrypi/libcamera fork so this MR adds it.

For the new apps to work:

• legacy camera stack firmware must not be loaded (no start_x in /boot/config.txt or /boot/usercfg.txt),
• udev must be available (setup-devd udev),
• camera_auto_detect=1 must be present in /boot/config.txt or /boot/usercfg.txt in order to load an appropriate dtoverlay,
• gpu_mem directives should be replaced in /boot/config.txt or /boot/usercfg.txt with setting the CMA pool: dtoverlay=vc4-kms-v3d,cma-256 (for 256M),
• some cameras need kernel >= 6.6.12 to work (for example imx477 works on 6.6.4 but imx219 needs 6.6.12). kernel 6.6.12 is now part of alpine 3.19.1