libcamera: controls: Expose string controls as std::string_view

When retrieving the value from a `ControlValue` usually one of two
things happen: a small, trivially copyable object is returned by
value; or a view into the internal buffer is provided. This is true
for everything except strings, which are returned in `std::string`,
incurring the overhead of string construction.

To guarantee no potentially "expensive" copies, use `std::string_view`
pointing to the internal buffer to return the value. This is similar
to how other array-like types are returned with a `Span<>`.

This is an API break, but its scope is limited to just `properties::Model`.

Bug: https://bugs.libcamera.org/show_bug.cgi?id=256
Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/apps/common/dng_writer.cpp

@@ -564,7 +564,7 @@ int DNGWriter::write(const char *filename, const Camera *camera,
 
 	const auto &model = cameraProperties.get(properties::Model);
 	if (model) {
-		TIFFSetField(tif, TIFFTAG_MODEL, model->c_str());
+		TIFFSetField(tif, TIFFTAG_MODEL, std::string(*model).c_str());
 		/* \todo set TIFFTAG_UNIQUECAMERAMODEL. */
 	}