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.

android/CMakeLists.txt

@@ -71,6 +71,7 @@ add_library(
 	../engine/node_panel_layer.cpp
 	../engine/node_panel_stroke.cpp
 	../engine/node_popup_menu.cpp
+	../engine/node_progress_bar.cpp
 	../engine/node_settings.cpp
 	../engine/node_slider.cpp
 	../engine/node_stroke_preview.cpp
@@ -82,6 +83,7 @@ add_library(
 
 target_include_directories(native-lib PRIVATE
 	${ANDROID_NDK}/sources/android/native_app_glue
+	src/main/cpp
 	../engine
 	../libs/glm
 	../libs/tinyxml2