Platform: fix EmscriptenApplication::MouseMoveEvent button numbering.

This all looked obviously correct so I never questioned it, but the spec
itself has the order mixed up for an unexplainable reason so it doesn't
match between MouseEvent and MouseMoveEvent.

doc/changelog.dox

@@ -1200,6 +1200,10 @@ See also:
     text. Now it emits both, consistently with @ref Platform::Sdl2Application
     and @ref Platform::GlfwApplication, and in the same order as in those. See
     [mosra/magnum#637](https://github.com/mosra/magnum/issues/637).
+-   Fixed swapped middle and right buttons in
+    @ref Platform::EmscriptenApplication::MouseMoveEvent, caused by the
+    JavaScript events themselves having an unexplainable inconsistency in
+    button numbering
 
 @subsection changelog-latest-deprecated Deprecated APIs
 

src/Magnum/Platform/EmscriptenApplication.h

@@ -1561,9 +1561,10 @@ class EmscriptenApplication::MouseEvent: public EmscriptenApplication::InputEven
          * @see @ref button()
          */
         enum class Button: std::int32_t {
-            Left,       /**< Left mouse button */
-            Middle,     /**< Middle mouse button */
-            Right       /**< Right mouse button */
+            /* https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/button */
+            Left = 0,       /**< Left mouse button */
+            Middle = 1,     /**< Middle mouse button */
+            Right = 2       /**< Right mouse button */
         };
 
         /** @brief Button */
@@ -1599,14 +1600,13 @@ class EmscriptenApplication::MouseMoveEvent: public EmscriptenApplication::Input
          * @see @ref buttons()
          */
         enum class Button: Int {
-            /** Left mouse button */
-            Left = 1 << 0,
+            /* https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/buttons,
+               note that Middle and Right has order swapped compared to
+               button / MouseEvent::Button, for some unexplainable reason */
 
-            /** Middle mouse button */
-            Middle = 1 << 1,
-
-            /** Right mouse button */
-            Right = 1 << 2
+            Left = 1 << 0,      /** Left mouse button */
+            Middle = 1 << 2,    /** Middle mouse button */
+            Right = 1 << 1      /** Right mouse button */
         };
 
         /**