Limit directional shadow draw distance, fixes #559, optimization (#1991)

* Shadow fadeout exponent hardcoded for now, should be user configurable.
* optimization - skip shadows outside visible range
(cherry picked from commit 0b12ebba1143888ee76302c33bbf67508717f42f)

drivers/gles2/rasterizer_gles2.cpp

@@ -5457,9 +5457,16 @@ void RasterizerGLES2::_setup_light(uint16_t p_light) {
 	}
 
 
-	light_data[VL_LIGHT_ATTENUATION][0]=l->vars[VS::LIGHT_PARAM_ENERGY];
-	light_data[VL_LIGHT_ATTENUATION][1]=l->vars[VS::LIGHT_PARAM_RADIUS];
-	light_data[VL_LIGHT_ATTENUATION][2]=l->vars[VS::LIGHT_PARAM_ATTENUATION];
+	light_data[VL_LIGHT_ATTENUATION][0] = l->vars[VS::LIGHT_PARAM_ENERGY];
+
+	if (l->type == VS::LIGHT_DIRECTIONAL) {
+		light_data[VL_LIGHT_ATTENUATION][1] = l->directional_shadow_param[VS::LIGHT_DIRECTIONAL_SHADOW_PARAM_MAX_DISTANCE];
+	}
+	else{
+		light_data[VL_LIGHT_ATTENUATION][1] = l->vars[VS::LIGHT_PARAM_RADIUS];
+	}
+
+	light_data[VL_LIGHT_ATTENUATION][2] = l->vars[VS::LIGHT_PARAM_ATTENUATION];
 
 	light_data[VL_LIGHT_SPOT_ATTENUATION][0]=Math::cos(Math::deg2rad(l->vars[VS::LIGHT_PARAM_SPOT_ANGLE]));
 	light_data[VL_LIGHT_SPOT_ATTENUATION][1]=l->vars[VS::LIGHT_PARAM_SPOT_ATTENUATION];

drivers/gles2/shaders/material.glsl

@@ -976,6 +976,12 @@ FRAGMENT_SHADER_CODE
 #ifdef LIGHT_USE_SHADOW
 #ifdef LIGHT_TYPE_DIRECTIONAL
 
+	float shadow_fade_exponent=5.0;  //hardcoded for now
+	float shadow_fade=pow(length(vertex_interp)/light_attenuation.g,shadow_fade_exponent);
+
+// optimization - skip shadows outside visible range
+	if(shadow_fade<1.0){
+
 #ifdef LIGHT_USE_PSSM
 
 
@@ -1101,6 +1107,12 @@ FRAGMENT_SHADER_CODE
 
 	shadow_attenuation=SAMPLE_SHADOW_TEX(shadow_coord.xy,shadow_coord.z);
 #endif
+
+	shadow_attenuation=mix(shadow_attenuation,1.0,shadow_fade);
+	}else{
+	shadow_attenuation=1.0;
+	};
+
 #endif
 
 #ifdef LIGHT_TYPE_OMNI