refactor: Migrate ViewportWidget to shader-based OpenGL rendering (FFP disabled)

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/shaders/simple.frag

@@ -0,0 +1,10 @@
+#version 330 core
+
+out vec4 FragColor;
+
+uniform vec4 objectColor;
+
+void main()
+{
+    FragColor = objectColor;
+}