Fix several clazy issue:

* Using copy-ctor but class SbMatrix/SbViewportRegion has a trivial copy-ctor but non trivial assign operator [-Wclazy-rule-of-two-soft]

src/Gui/SoFCOffscreenRenderer.cpp

@@ -423,7 +423,7 @@ void SoQtOffscreenRenderer::init(const SbViewportRegion & vpr,
     }
 
     this->didallocation = glrenderaction ? false : true;
-    this->viewport = vpr;
+    this->viewport = vpr; // clazy:exclude=rule-of-two-soft
 
     this->framebuffer = nullptr;
     this->numSamples = -1;
@@ -473,7 +473,7 @@ SoQtOffscreenRenderer::~SoQtOffscreenRenderer()
 void
 SoQtOffscreenRenderer::setViewportRegion(const SbViewportRegion & region)
 {
-    PRIVATE(this)->viewport = region;
+    PRIVATE(this)->viewport = region; // clazy:exclude=rule-of-two-soft
 }
 
 /*!