Support shadow maps in GLES

jme3-core/src/main/resources/Common/ShaderLib/GLSLCompat.glsllib

@@ -25,6 +25,7 @@ out vec4 outFragColor;
 #  define texture3D texture
 #  define textureCube texture
 #  define texture2DLod textureLod
+#  define shadow2D(a,b)  vec4(texture(a,b))
 #  define textureCubeLod textureLod
 #  if defined VERTEX_SHADER
 #    define varying out