3D views: use the correct resolution on high DPI displays!

Nowadays Qt gives us size in device independent pixels, and we were
using those values for the textures as well - but that means on high DPI
screens where device pixel ratio is 2, we were rendering in low resolution
and then upscaling the textures. This should fix that, and intermediate
textures use the correct size - the rendered output is much crispier, yay!

src/3d/qgswindow3dengine.cpp

@@ -99,7 +99,7 @@ void QgsWindow3DEngine::setSize( QSize s )
 
   mMapCanvas3D->setWidth( mSize.width() );
   mMapCanvas3D->setHeight( mSize.height() );
-  mFrameGraph->setSize( mSize );
+  mFrameGraph->setSize( mSize * mMapCanvas3D->devicePixelRatio() );
   camera()->setAspectRatio( float( mSize.width() ) / float( mSize.height() ) );
   emit sizeChanged();
 }