RawImageWidget.py : fix port to QOpenGLWidget

Qt5 docs specify differences between QGLWidget and QOpenGLWidget:
"...when invoking paintGL().
 QOpenGLWidget sets up the viewport via glViewport().
 It does not perform any clearing."

pyqtgraph/widgets/RawImageWidget.py

@@ -134,6 +134,8 @@ if HAVE_OPENGL:
             glDisable(GL_TEXTURE_2D)
 
         def paintGL(self):
+            glClear(GL_COLOR_BUFFER_BIT)
+
             if self.image is None:
                 if self.opts is None:
                     return
@@ -144,9 +146,6 @@ if HAVE_OPENGL:
             if not self.uploaded:
                 self.uploadTexture()
 
-            dpr = self.devicePixelRatio()
-            vp = (0, 0, int(self.width() * dpr), int(self.height() * dpr))
-            glViewport(*vp)
             glEnable(GL_TEXTURE_2D)
             glBindTexture(GL_TEXTURE_2D, self.texture)
             glColor4f(1, 1, 1, 1)