Platform: what the hell, these are not bitflags!

It's pure luck that this ever worked.

src/Magnum/Platform/GlfwApplication.h

@@ -586,7 +586,7 @@ class GlfwApplication::GLConfiguration {
          *
          * @see @ref Flags, @ref setFlags(), @ref Context::Flag
          */
-        enum class Flag: Int {
+        enum class Flag: UnsignedByte {
             #if defined(DOXYGEN_GENERATING_OUTPUT) || defined(GLFW_CONTEXT_NO_ERROR)
             /**
              * Specifies whether errors should be generated by the context.
@@ -595,11 +595,11 @@ class GlfwApplication::GLConfiguration {
              *
              * @note Supported since GLFW 3.2.
              */
-            NoError = GLFW_CONTEXT_NO_ERROR,
+            NoError = 1 << 1,
             #endif
 
-            Debug = GLFW_OPENGL_DEBUG_CONTEXT,  /**< Debug context */
+            Debug = 1 << 2,     /**< Debug context */
-            Stereo = GLFW_STEREO,               /**< Stereo rendering */
+            Stereo = 1 << 3     /**< Stereo rendering */
         };
 
         /**