GPUTest: Add support to test on Windows.

On windows the OpenGL context wasn't activated when created, on Linux it
is. This patch will activate the context in gpu/draw test cases.

source/blender/gpu/tests/gpu_testing.cc

@@ -18,6 +18,7 @@ void GPUTest::SetUp()
   CLG_init();
   ghost_system = GHOST_CreateSystem();
   ghost_context = GHOST_CreateOpenGLContext(ghost_system, glSettings);
+  GHOST_ActivateOpenGLContext(ghost_context);
   context = GPU_context_create(nullptr);
   GPU_init();
 }