Platform: fix Sdl2Application::setWindowIcon() on SDL 2.23+.

I was abusing the API and passing a negative pitch there to not have to
invert the image by hand. It stopped working in 2.23 when they hardened
the argument checking and, while working correctly, this feature was
accidental and undocumented.

Unfortunately it broke silently, because the API returned nullptr and
SDL_SetWindowIcon(..., nullptr) is then resetting an icon to nothing. So
I'm adding an internal assertion there now. Hopefully it doesn't start
blowing up for some reason again, heh.

src/Magnum/Platform/Sdl2Application.cpp

@@ -44,8 +44,10 @@
 #include <emscripten/emscripten.h>
 #include <emscripten/html5.h>
 #endif
+#include <Corrade/Utility/Algorithms.h>
 #include <Corrade/Utility/Arguments.h>
 
+#include "Magnum/Image.h"
 #include "Magnum/ImageView.h"
 #include "Magnum/PixelFormat.h"
 #include "Magnum/Math/ConfigurationValue.h"
@@ -361,11 +363,18 @@ void Sdl2Application::setWindowIcon(const ImageView2D& image) {
             CORRADE_ASSERT_UNREACHABLE("Platform::Sdl2Application::setWindowIcon(): unexpected format" << image.format(), );
     }
 
-    /* Images are loaded with origin at bottom left, flip it to top left.
-       Fortunately SDL accepts negative stride, so we don't need to do an
-       expensive flip ourselves. */
-    Containers::StridedArrayView3D<const char> pixels = image.pixels().flipped<0>();
-    SDL_Surface* icon = SDL_CreateRGBSurfaceWithFormatFrom(const_cast<void*>(pixels.data()) , image.size().x(), image.size().y(), 32, pixels.stride()[0], format);
+    /* Images are loaded with origin at bottom left, flip it to top left. SDL
+       only accepted a negative stride until version 2.23.1 and commit
+        https://github.com/libsdl-org/SDL/commit/535fdc3adcdc08a193ab0d45540014fd536cf251
+       so we need to manually flip the image now */
+    /** @todo take ImageFlag::YUp into account once it exists */
+    Image2D flippedImage{PixelStorage{}.setAlignment(1), image.format(), image.size(), Containers::Array<char>{NoInit, std::size_t(image.size().product()*image.pixelSize())}};
+    const Containers::StridedArrayView3D<char> flippedPixels = flippedImage.pixels();
+    Utility::copy(image.pixels().flipped<0>(), flippedPixels);
+
+    SDL_Surface* const icon = SDL_CreateRGBSurfaceWithFormatFrom(const_cast<void*>(flippedPixels.data()) , flippedImage.size().x(), flippedImage.size().y(), 32, flippedPixels.stride()[0], format);
+    CORRADE_INTERNAL_ASSERT(icon);
+
     SDL_SetWindowIcon(_window, icon);
     SDL_FreeSurface(icon);
 }