pattern offset, invert and flip(xy)

src/brush.h

@@ -77,6 +77,7 @@ public:
     float m_pattern_scale = .25f;
     float m_pattern_brightness = 0.5f;
     float m_pattern_contrast = 0.5f;
+    bool m_pattern_rand_offset = false;
 
     bool load_tip(const std::string& path, const std::string& thumb);
     bool load_dual(const std::string& path, const std::string& thumb);