Platform: fix severe aliasing artifacts with defaults on Emscripten.

The default for Sdl2Application canvas size was 800x600, but the canvas
was sized to 640x480 by the default CSS style. Sorry about this.

doc/changelog.dox

@@ -441,6 +441,9 @@ namespace Magnum {
     @extension{ARB,direct_state_access} code path, causing crashes on Mesa
 -   Properly export @ref Framebuffer::DrawAttachment to avoid linker failures
     on Windows
+-   On Emscripten, @ref Platform::Sdl2Application was by default rendering a
+    800x600 canvas to an area of 640x480 pixels, causing severe aliasing.
+    Canvas is now by default sized to 640x480 to match the default CSS style.
 -   @ref Platform::WindowlessWglApplication is fixed to allow the GL context to
     be created and destroyed multiple times during application run
 -   @ref SceneGraph::Camera now works properly with @ref Double underlying type

src/Magnum/Platform/Sdl2Application.cpp

@@ -529,7 +529,9 @@ Sdl2Application::Configuration::Configuration():
     #if !defined(CORRADE_TARGET_EMSCRIPTEN) && !defined(CORRADE_TARGET_IOS)
     _title("Magnum SDL2 Application"),
     #endif
-    #ifndef CORRADE_TARGET_IOS
+    #ifdef CORRADE_TARGET_EMSCRIPTEN
+    _size{640, 480},
+    #elif !defined(CORRADE_TARGET_IOS)
     _size{800, 600},
     #else
     _size{}, /* SDL2 detects someting for us */