Add specular map and abstract loading textures

shaders/frag.glsl

@@ -2,7 +2,7 @@
 
 struct Material {
   sampler2D diffuse;
-  vec3 specular;
+  sampler2D specular;
   float shininess;
 };
 
@@ -35,7 +35,7 @@ void main() {
   vec3  diff_tex           = vec3(texture(material.diffuse, uv_coords));
   vec3  ambient            = light.ambient * diff_tex;
   vec3  diffuse            = light.diffuse * (diff * diff_tex);
-  vec3  specular           = light.specular * (spec * material.specular);
+  vec3  specular           = light.specular * (spec * vec3(texture(material.specular, uv_coords)));
 
   color = vec4(ambient + diffuse + specular, 1.0);
 };