Fix screen flickering when glClear is not called

VERSION.md

@@ -1,16 +1,19 @@
 Version history
 ======
 
-### v2.0.0
+### v2.0.1
 
 #### Breaking changes
 * `abcg::Application::run` now takes by value a unique pointer to `abcg::OpenGLWindow`. Since `std::unique_ptr` cannot be copied, the caller must either use `std::move` or pass the pointer as an rvalue. This makes clear the intent of transferring ownership of the pointer, which was not explicit in the previous version that takes an lvalue reference.
 * Support for running multiple windows has been dropped. Multiple windows weren't working properly on Emscripten builds and aren't used in the course.
+* `abcg::opengl::loadCubemap` now transforms the cube map textures to a right-handed coordinate system by default. A small loading overhead is incurred because some of the bitmaps are flipped vertically and horizontally. This behavior can be disabled by setting the (new) parameter `rightHandedSystem` to `false`.
+
+#### Bug Fixes
+* Fixed flickering effect when `glClear` is not called for each frame. For this fix to work, `abcg::OpenGLSettings::preserveWebGLDrawingBuffer` must be `true` even when building for desktop.
 
 #### Other changes
 * `abcg::Exception::OpenGL` now generates a string describing all OpenGL error messages returned by `glGetError` when there are multiple errors.
 * All OpenGL functions from OpenGL ES 2.0 and ES 3.0 can now be qualified with the `abcg` namespace (e.g. `abcg::glActiveTexture(0)`) for automatic error handling in debug builds. This is an alternative to the `GL_CHECK` macro used in many engines to check for errors before and after each GL call.
-* Now, by default, `abcg::opengl::loadCubemap` transforms the cube map textures to a right-handed coordinate system. This will add a small loading overhead because some of the bitmaps are flipped vertically and horizontally. This behavior can be disabled by setting the (new) parameter `rightHandedSystem` to `false`.
 * Updated external libraries (Dear ImGui v1.84; cppitertools v2.1; {fmt} 8.0.1).
 
 ### v1.0.0

abcg/abcg_openglwindow.cpp

@@ -566,7 +566,12 @@ void abcg::OpenGLWindow::initialize(std::string_view basePath) {
   SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, majorVersion);
   SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, minorVersion);
 
-  SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
+  if (m_openGLSettings.preserveWebGLDrawingBuffer) {
+    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 0);
+  } else {
+    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
+  }
+
   SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, m_openGLSettings.depthBufferSize);
   SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, m_openGLSettings.stencilSize);
   if (m_openGLSettings.samples > 0) {
@@ -696,7 +701,12 @@ void abcg::OpenGLWindow::paint() {
   ImGui::Render();
   paintGL();
   ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
-  SDL_GL_SwapWindow(m_window);
+
+  if (m_openGLSettings.preserveWebGLDrawingBuffer) {
+    glFinish();
+  } else {
+    SDL_GL_SwapWindow(m_window);
+  }
 
   // Cap to 480 Hz
   if (m_deltaTime.elapsed() >= 1.0 / 480.0) {