Fix skid mark color

We convert the vertex color value to srgb, optimally the vertex color
should already be corrected on cpu side but it would break fallback
colors...

data/shaders/transparent.frag

@@ -12,7 +12,7 @@ varying vec2 uv;
 
 void main()
 {
-    vec4 Color = texture(tex, uv) * color;
+    vec4 Color = texture(tex, uv) * pow(color, vec4(2.2));
     // Premultiply alpha
     FragColor = vec4(Color.rgb * Color.a, Color.a);
 }

data/shaders/transparentfog.frag

@@ -21,7 +21,7 @@ varying vec2 uv;
 
 void main()
 {
-	vec4 diffusecolor = texture(tex, uv) * color;
+	vec4 diffusecolor = texture(tex, uv) * pow(color, vec4(2.2));
 	vec3 tmp = vec3(gl_FragCoord.xy / screen, gl_FragCoord.z);
 	tmp = 2. * tmp - 1.;