Add dpi option for save offscreen picture

.gitignore

src/manager/OffScreenPlotManager.cpp

src/plot/offscreen/OffScreenEmptyPlot.cpp

src/plot/offscreen/OffScreenEmptyPlot.h

src/plot/offscreen/OffScreenPlot.cpp

src/plot/offscreen/OffScreenPlot.h

src/plot/offscreen/OffScreenPlot1D.cpp

src/plot/offscreen/OffScreenPlot1D.h

src/plot/offscreen/OffScreenPlot2D.cpp

src/plot/offscreen/OffScreenPlot2D.h

src/view/mpl/Mpl.cpp

src/view/mpl/Mpl.h