Use circular fade instead of linear fade for distance fade

This makes distance fade look the same regardless of the camera angle,
for all distance fade modes (Pixel Alpha, Pixel Dither, Object Dither).
Distance fade now behaves like fog in this regard.

scene/resources/material.cpp

@@ -1252,15 +1252,16 @@ void BaseMaterial3D::_update_shader() {
 	}
 
 	if (distance_fade != DISTANCE_FADE_DISABLED) {
+		// Use the slightly more expensive circular fade (distance to the object) instead of linear
+		// (Z distance), so that the fade is always the same regardless of the camera angle.
 		if ((distance_fade == DISTANCE_FADE_OBJECT_DITHER || distance_fade == DISTANCE_FADE_PIXEL_DITHER)) {
 			if (!RenderingServer::get_singleton()->is_low_end()) {
 				code += "	{\n";
 
 				if (distance_fade == DISTANCE_FADE_OBJECT_DITHER) {
-					code += "		float fade_distance = abs((VIEW_MATRIX * MODEL_MATRIX[3]).z);\n";
-
+					code += "		float fade_distance = length((VIEW_MATRIX * MODEL_MATRIX[3]));\n";
 				} else {
-					code += "		float fade_distance = -VERTEX.z;\n";
+					code += "		float fade_distance = length(VERTEX);\n";
 				}
 				// Use interleaved gradient noise, which is fast but still looks good.
 				code += "		const vec3 magic = vec3(0.06711056f, 0.00583715f, 52.9829189f);";
@@ -1274,7 +1275,7 @@ void BaseMaterial3D::_update_shader() {
 			}
 
 		} else {
-			code += "	ALPHA*=clamp(smoothstep(distance_fade_min,distance_fade_max,-VERTEX.z),0.0,1.0);\n";
+			code += "	ALPHA *= clamp(smoothstep(distance_fade_min, distance_fade_max, length(VERTEX)), 0.0, 1.0);\n";
 		}
 	}