implement stencil from file

src/node_stroke_preview.cpp

@@ -80,7 +80,7 @@ void NodeStrokePreview::draw_stroke()
         tex.bind();
         m_sampler_brush.bind(0);
         
-        auto& stencil = TextureManager::get(const_hash("data/paper.jpg"));
+        auto& stencil = TextureManager::get(b.m_tex_stencil_id);
         glActiveTexture(GL_TEXTURE1);
         stencil.bind();
         m_sampler.bind(1);