implement oscillating screensaver

this implementation avoids the display content jumping the full
screensaver offset from right to left when the modulo operator
wraps. this change makes the display content walk from right to
left as it did walk from left to right.

src/Display_Graphic.cpp

@@ -103,27 +103,23 @@ void DisplayGraphicClass::printText(const char* text, const uint8_t line)
     if (!_isLarge) {
         dispX = (line == 0) ? 5 : 0;
     } else {
-        switch (line) {
-        case 0:
-            if (_diagram_mode == DiagramMode_t::Small) {
-                // Center between left border and diagram
-                dispX = (CHART_POSX - _display->getStrWidth(text)) / 2;
-            } else {
-                // Center on screen
-                dispX = (_display->getDisplayWidth() - _display->getStrWidth(text)) / 2;
-            }
-            break;
-        case 3:
+        if (line == 0 && _diagram_mode == DiagramMode_t::Small) {
+            // Center between left border and diagram
+            dispX = (CHART_POSX - _display->getStrWidth(text)) / 2;
+        } else {
             // Center on screen
             dispX = (_display->getDisplayWidth() - _display->getStrWidth(text)) / 2;
-            break;
-        default:
-            dispX = 5;
-            break;
         }
     }
 
-    dispX += enableScreensaver ? (_mExtra % 7) : 0;
+    if (enableScreensaver) {
+        unsigned maxOffset = (_isLarge?8:6);
+        unsigned period = 2 * maxOffset;
+        unsigned step = _mExtra % period;
+        int offset = (step <= maxOffset)?step:(period - step);
+        offset -= (_isLarge?5:0); // oscillate around center on large screens
+        dispX += offset;
+    }
     _display->drawStr(dispX, _lineOffsets[line], text);
 }