libcamera: controls: Add zero-copy set API for ControlValue

Extend the ControlValue class with a reserve() function to set the value
without actually copying data, and a non-const data() function that
allows writing data directly to the ControlValue storage. This allows
allocating memory directly in ControlValue, potentially removing a data
copy.

Note that this change was implemented before ByteStreamBuffer gained the
zero-copy read() variant, and doesn't actually save a copy in the
control serializer. It however still simplifies
ControlSerializer::loadControlValue().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/include/control_serializer.h

@@ -40,9 +40,6 @@ private:
 	static void store(const ControlValue &value, ByteStreamBuffer &buffer);
 	static void store(const ControlInfo &info, ByteStreamBuffer &buffer);
 
-	template<typename T>
-	ControlValue loadControlValue(ByteStreamBuffer &buffer, bool isArray,
-				      unsigned int count);
 	ControlValue loadControlValue(ControlType type, ByteStreamBuffer &buffer,
 				      bool isArray = false, unsigned int count = 1);
 	ControlInfo loadControlInfo(ControlType type, ByteStreamBuffer &buffer);