fix #20; RectMask2D doesn't work on WebGL

UIAdditive.shader

@@ -54,7 +54,6 @@
             #include "UnityCG.cginc"
             #include "UnityUI.cginc"
 
-            #pragma multi_compile __ UNITY_UI_CLIP_RECT
             #pragma multi_compile __ UNITY_UI_ALPHACLIP
 
             struct appdata_t
@@ -97,10 +96,7 @@
             fixed4 frag(v2f IN) : SV_Target
             {
                 half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;
-
-                #ifdef UNITY_UI_CLIP_RECT
                 color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
-                #endif
 
                 #ifdef UNITY_UI_ALPHACLIP
                 clip (color.a - 0.001);