Platform: fix Sdl2Application::setSwapInterval() with late swap behavior.

For quite a while, setSwapInterval() was reporting that "swap interval
was ignored by the driver". Since I used to have that behavior ages ago
on a NVidia Optimus machine (where it was just *impossible* to have
VSync, imagine that!!), I assumed it was a similar wart in Mesa and
didn't bother looking into it.

It turns out, however, that calling setSwapInterval(1) may result in
SDL_GL_GetSwapInterval() returning -1 instead of 1, thus helpfully
enabling late-swap behavior for me. Since -1 != -1, the code treated
that the same as if SDL_GL_GetSwapInterval() returned 0 (which was the
case with NV Optimus having broken VSync), but it's not an error in
fact.

doc/changelog.dox

@@ -725,6 +725,9 @@ See also:
     case as for a WebGL-enabled context
 -   Fixed a crash in @ref Platform::GlfwApplication::setCursor() on the
     upcoming GLFW 3.4
+-   Fixed @ref Platform::Sdl2Application::setSwapInterval() to take late swap
+    behavior (@cpp -1 @ce) into account instead of treating it as an error
+    and continuing with timer-based framerate capping
 -   @ref SceneGraph::BasicMatrixTransformation2D,
     @ref SceneGraph::BasicMatrixTransformation3D,
     @ref SceneGraph::BasicRigidMatrixTransformation2D and

src/Magnum/Platform/Sdl2Application.cpp

@@ -781,8 +781,11 @@ bool Sdl2Application::setSwapInterval(const Int interval) {
         return false;
     }
 
-    if(SDL_GL_GetSwapInterval() != interval) {
-        Error() << "Platform::Sdl2Application::setSwapInterval(): swap interval setting ignored by the driver";
+    /* Setting interval to 1 may cause SDL_GL_GetSwapInterval() to return -1,
+       which is a valid case */
+    const Int actualInterval = SDL_GL_GetSwapInterval();
+    if(actualInterval != interval && !(interval == 1 && actualInterval == -1)) {
+        Error() << "Platform::Sdl2Application::setSwapInterval(): swap interval setting ignored by the driver:" << SDL_GetError();
         _flags &= ~Flag::VSyncEnabled;
         return false;
     }