-improved WASM compilation scripts

-added external bindbc.sdl import for WASM
-working on demos (WIP, working simple demo with ECS and SDL2)
-small change in ecs.std

demos/external/imports/bindbc/sdl/bind/sdlerror.d

@@ -0,0 +1,28 @@
+
+//          Copyright Michael D. Parker 2018.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+module bindbc.sdl.bind.sdlerror;
+
+version(BindSDL_Static) {
+    extern(C) @nogc nothrow {
+        void SDL_SetError(const(char)*,...);
+        const(char)* SDL_GetError();
+        void SDL_ClearError();
+    }
+}
+else {
+    extern(C) @nogc nothrow {
+        alias pSDL_SetError = void function(const(char)*,...);
+        alias pSDL_GetError = const(char)* function();
+        alias pSDL_ClearError = void function();
+    }
+
+    __gshared {
+        pSDL_SetError SDL_SetError;
+        pSDL_GetError SDL_GetError;
+        pSDL_ClearError SDL_ClearError;
+    }
+}