ipa: rpi: Add HDR support

Add support for the following HDR modes in the Raspberry Pi IPA:
- Night mode
- Single exposure mode
- Multi-exposure (merged and unmerged)

The algorithm is updated to expect the HDR short channel to meter
explicitly for highlights. This means that it will not in general
under-expose the short channel more than is actually necessary.

When images don't have much saturation, it's good to detect this so
that some of the boost we want to apply to the dark areas can be
implemented as regular gain. This means we can then adjust the tone
curve less, leading to less flat looking images.

The impact on the HDR algorithm is then that this determines how we
build tonemaps dynamically. The highlights are more-or-less correct
now, so we have to build a power-type curve that gives us the
appropriately configured targets in the lower part of the histogram.

We allow the tuning file to supply the maximum spatial gain value,
rather than the whole curve (though it can supply this if it
wants). Some parameter defaults are tweaked to be generally better
across the range of our cameras.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Acked-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

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