CustomAttribPass now may exports a vector field as a 2D texture

src/DemoApp/main.cpp

src/libRender/RadiumNBR/Gui/VisualizationGui.cpp

src/libRender/RadiumNBR/NodeBasedRenderer.cpp

src/libRender/RadiumNBR/Passes/ClearPass.cpp

src/libRender/RadiumNBR/Passes/CustomAttribToColorPass.cpp

src/libRender/RadiumNBR/Passes/CustomAttribToColorPass.hpp

src/libRender/RadiumNBR/Renderer/Visualization.cpp

src/libRender/RadiumNBR/Renderer/Visualization.hpp