fix initialization opengl glitch

src/framework/graphics/graphics.cpp

@@ -86,6 +86,10 @@ void Graphics::init()
     glEnable(GL_CULL_FACE);
     */
 
+    // clear color buffer
+    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
+    glClear(GL_COLOR_BUFFER_BIT);
+
     // determine max texture size
     GLint maxTextureSize = 0;
     glGetIntegerv(GL_MAX_TEXTURE_SIZE, &maxTextureSize);