Platform: use zero window size on iOS and let SDL2 autodetect.

Hopefully this feature won't go away in some future version.

src/Magnum/Platform/Sdl2Application.cpp

@@ -439,7 +439,12 @@ Sdl2Application::Configuration::Configuration():
     #if !defined(CORRADE_TARGET_EMSCRIPTEN) && !defined(CORRADE_TARGET_IOS)
     _title("Magnum SDL2 Application"),
     #endif
-    _size(800, 600), _windowFlags{}, _sampleCount(0)
+    #ifndef CORRADE_TARGET_IOS
+    _size{800, 600},
+    #else
+    _size{}, /* SDL2 detects someting for us */
+    #endif
+    _windowFlags{}, _sampleCount(0)
     #ifndef CORRADE_TARGET_EMSCRIPTEN
     , _version(Version::None), _sRGBCapable{false}
     #endif

src/Magnum/Platform/Sdl2Application.h

@@ -653,7 +653,10 @@ class Sdl2Application::Configuration {
          * @brief Set window size
          * @return Reference to self (for method chaining)
          *
-         * Default is `{800, 600}`.
+         * Default is `{800, 600}`. On iOS it defaults to a "reasonable" size
+         * based on whether HiDPI support is enabled using
+         * @ref WindowFlag::AllowHighDpi, but not necessarily native display
+         * resolution (you have to set it explicitly).
          */
         Configuration& setSize(const Vector2i& size) {
             _size = size;