add fog_curve parameter & update shaders

res/shaders/entity.glslf

@@ -1,7 +1,7 @@
 in vec4 a_color;
 in vec2 a_texCoord;
-in float a_distance;
 in vec3 a_dir;
+in float a_fog;
 out vec4 f_color;
 
 uniform sampler2D u_texture0;
@@ -14,12 +14,10 @@ uniform bool u_alphaClip;
 void main() {
     vec3 fogColor = texture(u_cubemap, a_dir).rgb;
     vec4 tex_color = texture(u_texture0, a_texCoord);
-    float depth = (a_distance/256.0);
     float alpha = a_color.a * tex_color.a;
     // anyway it's any alpha-test alternative required
     if (alpha < (u_alphaClip ? 0.5f : 0.15f))
         discard;
-    f_color = mix(a_color * tex_color, vec4(fogColor,1.0), 
-              min(1.0, pow(depth*u_fogFactor, u_fogCurve)));
+    f_color = mix(a_color * tex_color, vec4(fogColor,1.0), a_fog);
     f_color.a = alpha;
 }