Clearing canvas buffer in ArduinoLEDMatrix::clear() in addition to frame buffer (#335)

* Also clearing canvas buffer in ArduinoLEDMatrix::clear()

Author: stefanmarks

Diff for: libraries/Arduino_LED_Matrix/src/Arduino_LED_Matrix.h

@@ -260,6 +260,9 @@ class ArduinoLEDMatrix
         	0x00000000
         };
         loadFrame(fullOff);
+#ifdef MATRIX_WITH_ARDUINOGRAPHICS
+        memset(_canvasBuffer, 0, sizeof(_canvasBuffer));
+#endif
     }