Platform: default to non-resizable window in Sdl2Application.

The user is not required to provide viewportEvent() implementation, thus
it's not wise to allow resizing by default.

src/Magnum/Platform/Sdl2Application.cpp

@@ -334,7 +334,7 @@ Sdl2Application::Configuration::Configuration():
     #ifndef CORRADE_TARGET_EMSCRIPTEN
     _title("Magnum SDL2 Application"),
     #endif
-    _size(800, 600), _windowFlags(WindowFlag::Resizable), _sampleCount(0)
+    _size(800, 600), _windowFlags{}, _sampleCount(0)
     #ifndef CORRADE_TARGET_EMSCRIPTEN
     , _version(Version::None)
     #endif

src/Magnum/Platform/Sdl2Application.h

@@ -530,7 +530,7 @@ class Sdl2Application::Configuration {
          * @brief Set window flags
          * @return Reference to self (for method chaining)
          *
-         * Default is @ref WindowFlag::Resizable.
+         * Default are none.
          */
         Configuration& setWindowFlags(WindowFlags flags) {
             _windowFlags = flags;