Move shaders to separate files

resources/shaders/gl_solid_color.frag

@@ -0,0 +1,11 @@
+#version 330 core
+in  vec3 v_normal;
+out vec4 color;
+uniform vec3 light_dir;
+
+void main() {
+    float brightness = dot(normalize(v_normal), normalize(light_dir));
+    vec3 dark_color = vec3(0.6, 0.0, 0.0);
+    vec3 regular_color = vec3(1.0, 0.0, 0.0);
+    color = vec4(mix(dark_color, regular_color, brightness), 1.0);
+}

resources/shaders/gl_solid_color.vert

@@ -0,0 +1,12 @@
+#version 330 core
+in  vec3 position;
+in  vec3 normal;
+uniform mat4 model;
+uniform mat4 view;
+uniform mat4 projection;
+out vec3 v_normal;
+void main() {
+    mat4 modelview = view * model;
+    v_normal = transpose(inverse(mat3(modelview))) * normal;
+    gl_Position = projection * modelview * vec4(position, 1.0);
+}