implement multithreaded rendering with context switch, gl state save/restore, add progress bar ui node, implement stencil texture for brush, implement multithreaded canvas load/save/export pano. Missing multithread in windows.

engine/node_progress_bar.cpp

@@ -0,0 +1,24 @@
+#include "pch.h"
+#include "log.h"
+#include "node_progress_bar.h"
+#include "layout.h"
+
+Node* NodeProgressBar::clone_instantiate() const
+{
+    return new NodeProgressBar();
+}
+
+void NodeProgressBar::init()
+{
+    auto tpl = static_cast<const NodeBorder*>(init_template("progress-bar"));
+    m_color = tpl->m_color;
+    m_border_color = tpl->m_border_color;
+    m_thinkness = tpl->m_thinkness;
+
+    m_title = find<NodeText>("title");
+    btn_cancel = find<NodeButton>("btn-cancel");
+    btn_cancel->on_click = [&](Node*) { destroy(); };
+    m_progress = find<NodeBorder>("progress");
+    
+    m_progress->SetWidthP(10);
+}