Platform: deprecate Sdl2Application AllowHighDpi window flag.

It's now passed implicitly on platforms that need it.

doc/changelog.dox

@@ -281,6 +281,9 @@ See also:
     because it doesn't provide all information needed for DPI-aware rendering.
     Override the @ref Platform::Sdl2Application::viewportEvent(ViewportEvent&) "viewportEvent(ViewportEvent&)"
     function instead.
+-   `Platform::Sdl2Application::Configuration::WindowFlags::AllowHighDpi` is
+    deprecated and has no effect as it is now passed implicitly on platforms
+    that need it
 -   `Math::Geometry`, `Math::Geometry::Distance` and
     `Math::Geometry::Intersection` namespaces are deprecated for being too
     deeply nested, use @ref Math::Distance and @ref Math::Intersection instead
@@ -1065,7 +1068,7 @@ a high-level overview.
     application) (see [mosra/magnum#105](https://github.com/mosra/magnum/issues/105))
 -   Added @ref Platform::Sdl2Application::GLConfiguration::setSRGBCapable() "Platform::Sdl2Application::Configuration::setSRGBCapable()"
 -   Added @ref Platform::Sdl2Application::Configuration::WindowFlag::Borderless
-    and @ref Platform::Sdl2Application::Configuration::WindowFlag::AllowHighDpi
+    and `Platform::Sdl2Application::Configuration::WindowFlag::AllowHighDpi`
     for iOS and macOS
 -   Added @ref Platform::WindowlessGlxApplication::Configuration::setFlags() "Platform::Windowless*Application::Configuration::setFlags()" with
     @ref Platform::WindowlessGlxApplication::Configuration::Flag::Debug "Flag::Debug"

src/Magnum/Platform/Sdl2Application.cpp

@@ -241,7 +241,7 @@ bool Sdl2Application::tryCreate(const Configuration& configuration) {
         #endif
         SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
         scaledWindowSize.x(), scaledWindowSize.y(),
-        Uint32(configuration.windowFlags()&~Configuration::WindowFlag::Contextless))))
+        SDL_WINDOW_ALLOW_HIGHDPI|Uint32(configuration.windowFlags() & ~Configuration::WindowFlag::Contextless))))
     {
         Error() << "Platform::Sdl2Application::tryCreate(): cannot create window:" << SDL_GetError();
         return false;
@@ -365,7 +365,7 @@ bool Sdl2Application::tryCreate(const Configuration& configuration, const GLConf
         #endif
         SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
         scaledWindowSize.x(), scaledWindowSize.y(),
-        SDL_WINDOW_OPENGL|SDL_WINDOW_HIDDEN|Uint32(configuration.windowFlags()))))
+        SDL_WINDOW_OPENGL|SDL_WINDOW_HIDDEN|SDL_WINDOW_ALLOW_HIGHDPI|Uint32(configuration.windowFlags()))))
     {
         Error() << "Platform::Sdl2Application::tryCreate(): cannot create window:" << SDL_GetError();
         return false;
@@ -418,7 +418,7 @@ bool Sdl2Application::tryCreate(const Configuration& configuration, const GLConf
         if(!(_window = SDL_CreateWindow(configuration.title().data(),
             SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
             scaledWindowSize.x(), scaledWindowSize.y(),
-            SDL_WINDOW_OPENGL|SDL_WINDOW_HIDDEN|Uint32(configuration.windowFlags()&~Configuration::WindowFlag::Contextless))))
+            SDL_WINDOW_OPENGL|SDL_WINDOW_HIDDEN|SDL_WINDOW_ALLOW_HIGHDPI|Uint32(configuration.windowFlags()&~Configuration::WindowFlag::Contextless))))
         {
             Error() << "Platform::Sdl2Application::tryCreate(): cannot create window:" << SDL_GetError();
             return false;

src/Magnum/Platform/Sdl2Application.h

@@ -235,8 +235,6 @@ options needs to be set via a `*.plist` file. Some options can be configured
 from runtime when creating the SDL2 application window, see documentation of
 a particular value for details:
 
--   @ref Configuration::WindowFlag::AllowHighDpi allows creating HiDPI/Retina
-    drawable
 -   @ref Configuration::WindowFlag::Borderless hides the menu bar
 -   @ref Configuration::WindowFlag::Resizable makes the application respond to
     device orientation changes
@@ -1171,14 +1169,15 @@ class Sdl2Application::Configuration {
             /** No window decoration. On iOS this hides the menu bar. */
             Borderless = SDL_WINDOW_BORDERLESS,
 
-            #ifndef CORRADE_TARGET_EMSCRIPTEN
+            #ifdef MAGNUM_BUILD_DEPRECATED
             /**
-             * Allow high DPI. On iOS you also have to set the
-             * `NSHighResolutionCapable` entry in the `*.plist` file to make
-             * it working.
-             * @note Not available in @ref CORRADE_TARGET_EMSCRIPTEN "Emscripten".
+             * Allow high DPI.
+             *
+             * @deprecated Has no effect, as this flag is passed implicitly
+             *      on platforms where needed. See
+             *      @ref Platform-Sdl2Application-dpi for more information.
              */
-            AllowHighDpi = SDL_WINDOW_ALLOW_HIGHDPI,
+            AllowHighDpi CORRADE_DEPRECATED_ENUM("has no effect, passed implicitly on platforms that need it") = 0,
             #endif
 
             Hidden = SDL_WINDOW_HIDDEN,             /**< Hidden window */