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

src/brush.h

@@ -5,6 +5,9 @@
 
 class Brush
 {
+    std::shared_ptr<Image> m_tip_img;
+    std::shared_ptr<Image> m_pattern_img;
+    std::shared_ptr<Image> m_dual_img;
 public:
     //Brush() = default;
     //Brush(const Brush& brush) = default;
@@ -94,6 +97,9 @@ public:
     bool load_dual(const std::string& path, const std::string& thumb);
     bool load_pattern(const std::string& path, const std::string& thumb);
     bool load();
+    bool preload();
+    void unload();
+    bool valid();
 };
 
 struct StrokeSample