Add texture support to the renderer

- Refactor to use the new model::Model struct
- Implement blinn-phong shading
- Add texture support for shaders

resources/shaders/gl_textured.frag

@@ -0,0 +1,34 @@
+#version 330 core
+
+in  vec3 v_normal;
+in  vec2 v_tex;
+in  vec3 v_position;
+
+out vec4 frag_color;
+
+uniform vec3 u_light;
+uniform sampler2D tex;
+uniform vec3 color; // base colour factor (acts as solid colour when no texture)
+
+void main() {
+    // Combine base texture (or constant white) with colour factor supplied by CPU.
+    vec3 base_col = texture(tex, v_tex).rgb * color;
+
+    vec3 ambient_color  = base_col * 0.2;
+    vec3 diffuse_color  = base_col * 0.6;
+    vec3 specular_color = vec3(1.0);
+
+    // u_light is the direction **from the light towards the fragment**.
+    float diffuse = max(dot(normalize(v_normal), normalize(u_light)), 0.0);
+
+    vec3 camera_dir = normalize(-v_position);
+    vec3 half_dir   = normalize(normalize(u_light) + camera_dir);
+    float specular  = pow(max(dot(half_dir, normalize(v_normal)), 0.0), 16.0);
+
+    vec3 result = ambient_color + diffuse * diffuse_color + specular * specular_color;
+
+    // Convert from linear to sRGB for display (approximate γ-correction)
+    result = pow(result, vec3(1.0 / 2.2));
+
+    frag_color = vec4(result, 1.0);
+}