M2(OpenGL): GL mock backend (no loader) + mock-backed capture coverage

Add render_gl.cpp (selectable as `gl`): renders the animated pattern with
scissored clears (glClear + glScissor -- GL 1.1, exported straight from
opengl32) and presents with SwapBuffers. No glad/submodule needed: a legacy
wglCreateContext + <GL/gl.h> suffices, so the planned loader dependency was
dropped. GL is bottom-left origin and the capture flips top-down, so the
frame-counter block is drawn at the GL top to land top-left in the captured
image. Window class gains CS_OWNDC for a stable GL DC; best-effort vsync via a
runtime wglSwapIntervalEXT lookup.

The GL SwapBuffers/wglSwapBuffers hook now bumps the shared present counter
(it's the GL present), so present_calls works for GL games too. mock_game_test
decodes GL frames through the existing glReadPixels capture path; 15/15 ctest.
Roadmap: OpenGL milestone done and removed (Vulkan renumbered to M2);
architecture/lessons/test docs updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/mock_game_test.cpp

@@ -522,6 +522,7 @@ int main()
 		return 0;
 	}
 
+	test_video_capture("gl", device);
 	test_video_capture("dx9ex", device);
 	test_video_capture("dx9", device);
 	test_video_capture("dx10", device);