xrRenderGL: fixed crash when applying postprocess

For example, It happens when anomaly activates

res/gamedata/shaders/gl/postprocess.s

@@ -19,6 +19,6 @@ function l_special        (shader, t_base, t_second, t_detail)
 	shader:sampler	("s_base1")    	:texture("$user$albedo")	: clamp() : f_linear ()
 	shader:sampler	("s_noise")    	:texture("fx\\fx_noise2")	: f_linear ()
 
-	shader:sampler	("s_grad1")    	:texture("grad\\grad_red_yellow")	: clamp() : f_linear ()
-	shader:sampler	("s_grad1")    	:texture("grad\\grad_test1")	: clamp() : f_linear ()
+	shader:sampler	("s_grad0")    	:texture("$user$cmap0")	: clamp() : f_linear ()
+	shader:sampler	("s_grad1")    	:texture("$user$cmap1")	: clamp() : f_linear ()
 end