treewide: Remove top-level const from return types

Top-level `const` qualifiers are not useful, so avoid them. This is done
either by simply removing the top-level `const`, or making the function
return a reference to const where that is appropriate.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>

include/libcamera/internal/matrix.h

@@ -56,7 +56,7 @@ public:
 
 	~Matrix() = default;
 
-	const std::string toString() const
+	std::string toString() const
 	{
 		std::stringstream out;
 

include/libcamera/internal/v4l2_subdevice.h

@@ -66,7 +66,7 @@ struct V4L2SubdeviceFormat {
 	Size size;
 	std::optional<ColorSpace> colorSpace;
 
-	const std::string toString() const;
+	std::string toString() const;
 };
 
 std::ostream &operator<<(std::ostream &out, const V4L2SubdeviceFormat &f);

include/libcamera/internal/v4l2_videodevice.h

@@ -178,7 +178,7 @@ public:
 	std::array<Plane, 3> planes;
 	unsigned int planesCount = 0;
 
-	const std::string toString() const;
+	std::string toString() const;
 };
 
 std::ostream &operator<<(std::ostream &out, const V4L2DeviceFormat &f);