unload brush texture from main memory, preload brush on preview to free render thread

src/node_stroke_preview.cpp

@@ -487,10 +487,19 @@ void NodeStrokePreview::draw_stroke()
                 auto node = s_queue.Get();
                 if (node)
                 {
+                    // if the brush is not already loaded, load it and then destroy it
+                    bool to_unload = (node->m_brush->m_tip_texture == nullptr);
+                    node->m_brush->preload();
+
                     node->async_start();
                     gl_state gl;
                     gl.save();
+
+                    node->m_brush->load();
                     node->draw_stroke_immediate();
+                    if (to_unload)
+                        node->m_brush->unload();
+                    
                     gl.restore();
                     node->app_redraw();
                     node->async_end();