Scope: reset trigger and traces samples index when trace size changes. Fixes #927

sdrbase/dsp/scopevis.cpp

@@ -158,6 +158,8 @@ void ScopeVis::configure(
     {
         setTraceSize(traceSize);
         m_settings.m_traceLen = traceSize;
+        m_triggerState = TriggerUntriggered;
+        m_traces.resetControls();
     }
 
     if (m_timeBase != timeBase)

sdrbase/dsp/scopevis.h

@@ -1059,6 +1059,13 @@ private:
             }
         }
 
+        void resetControls()
+        {
+            for (auto traceControl : m_tracesControl) {
+                traceControl->reset();
+            }
+        }
+
     private:
         float *m_x0;
         float *m_x1;