Added SDL_GLcontextFlags enumeration.

The SDL_GLcontextFlags enumeration contains the values that are used in the SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, ...). This is required for enabling the debug or robustness OpenGL profiles.

src/SDL2.cs

@@ -905,6 +905,19 @@ namespace SDL2
 			SDL_GL_CONTEXT_PROFILE_ES				= 0x0004
 		}
 
+        /// <summary>
+        /// This enumeration is used in conjunction with SDL_GL_SetAttribute
+        /// and SDL_GL_CONTEXT_FLAGS. Multiple flags can be OR'd together. 
+        /// </summary>
+        [Flags]
+        public enum SDL_GLcontext
+        {
+            SDL_GL_CONTEXT_DEBUG_FLAG              = 0x0001,
+            SDL_GL_CONTEXT_FORWARD_COMPATIBLE_FLAG = 0x0002,
+            SDL_GL_CONTEXT_ROBUST_ACCESS_FLAG      = 0x0004,
+            SDL_GL_CONTEXT_RESET_ISOLATION_FLAG    = 0x0008
+        }
+		
 		/// <summary>
 		/// An enumeration of window events. 
 		/// </summary>