Opera's latest releases' rendering engine is still recorded as Presto but this is only true for Opera <= 12. Next version was Opera 15 and it uses Blink (Webkit) since then.
We are currently using an very old detection for browser engines. The new DeviceDetector will get an engine detection aswell. As soon as that is implemented, we will integrate that in Piwik, so detected engines will be up to date. See piwik/DeviceDetector#5263 Switching that will be done in #3962, so I'll close this ticket.
Added a tag "duplicate"