implement layer bounds optminize function, add history to draw_objects with the right bounds.

src/canvas.h

@@ -48,6 +48,29 @@ public:
                 std::fill_n(image[i].get(), width*height*4, 0);
             }
         }
+        void optimize()
+        {
+            for (int i = 0; i < 6; i++)
+            {
+                if (!m_dirty_face[i] || !image[i])
+                    continue;
+                auto data = reinterpret_cast<glm::i8vec4*>(image[i].get());
+                glm::ivec2 bbmin(width, height);
+                glm::ivec2 bbmax(0);
+                for (int y = 0; y < height; y++)
+                {
+                    for (int x = 0; x < width; x++)
+                    {
+                        if (data[x + y * width].a > 0)
+                        {
+                            bbmin = glm::min(bbmin, { x, y });
+                            bbmax = glm::max(bbmax, { x + 1, y + 1 });
+                        }
+                    }
+                }
+                glm::vec2 bbsz = bbmax - bbmin;
+            }
+        }
     };
     void resize(int width, int height);
     bool create(int width, int height, std::string name);
@@ -55,6 +78,7 @@ public:
     Snapshot snapshot();
     void restore(const Snapshot& snap);
     void destroy();
+    void optimize();
 };
 
 struct PPIThumb
@@ -155,6 +179,7 @@ public:
     static glm::mat4 m_plane_transform[6];
     glm::vec2 stencil_offset;
     Sampler m_sampler;
+    Sampler m_sampler_nearest;
     Sampler m_sampler_linear;
     Sampler m_sampler_brush;
     Sampler m_sampler_bg;