implement add and remove custom brushes

src/image.cpp

@@ -39,6 +39,11 @@ bool Image::load_file(std::string filename)
     return true;
 }
 
+bool Image::save(const std::string& path)
+{
+    return stbi_write_png(path.c_str(), width, height, comp, data(), 0);
+}
+
 void Image::flip()
 {
     auto flipped = std::make_unique<uint8_t[]>(width*height*4);