example: request stencil + MSAA buffers for GL contexts

Request a stencil buffer and multisample buffers for the SDL GL contexts in
Example and MultiCanvas so the direct back‑buffer path has the needed
stencil/multisample support.

Author: wenjieshen

src/Example.h

@@ -370,6 +370,9 @@ struct GlWindow : Window
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
+        SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8);
+        SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
+        SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 4);
 #endif
         window = SDL_CreateWindow("ThorVG Example (OpenGL/ES)", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, width, height, SDL_WINDOW_OPENGL | SDL_WINDOW_HIDDEN | SDL_WINDOW_RESIZABLE);
         context = SDL_GL_CreateContext(window);

src/MultiCanvas.cpp

@@ -183,6 +183,9 @@ void runGl()
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
+        SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8);
+        SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
+        SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 4);
 #endif
 
     auto window = SDL_CreateWindow("ThorVG Example (OpenGL)", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, WIDTH, HEIGHT, SDL_WINDOW_OPENGL | SDL_WINDOW_HIDDEN);