Fixing an issue with fxaa shader precision

Babylon/Shaders/fxaa.fragment.fx

@@ -1,4 +1,8 @@
-#define FXAA_REDUCE_MIN   (1.0/128.0)
+#ifdef GL_ES
+precision mediump float;
+#endif
+
+#define FXAA_REDUCE_MIN   (1.0/128.0)
 #define FXAA_REDUCE_MUL   (1.0/8.0)
 #define FXAA_SPAN_MAX     8.0