libcamera: utils: Add hex stream output helper

Add a utils::hex() function that simplifies writing hexadecimal values
to an ostream. The function handles the '0x' prefix, the field width and
the fill character automatically. Use it through the libcamera code
base, and add a test.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/v4l2_subdevice.cpp

@@ -21,6 +21,7 @@
 #include "log.h"
 #include "media_device.h"
 #include "media_object.h"
+#include "utils.h"
 
 /**
  * \file v4l2_subdevice.h
@@ -76,10 +77,7 @@ LOG_DECLARE_CATEGORY(V4L2)
 const std::string V4L2SubdeviceFormat::toString() const
 {
 	std::stringstream ss;
-
-	ss.fill(0);
-	ss << size.toString() << "-0x" << std::hex << std::setw(4) << mbus_code;
-
+	ss << size.toString() << "-" << utils::hex(mbus_code, 4);
 	return ss.str();
 }