building standalone compute shaders

src/shader/shader_builder.cpp

@@ -44,8 +44,14 @@ Shader ShaderBuilder::build()
 {
   // shader program
   GLuint program_id = glCreateProgram();
-  glAttachShader(program_id, vertex_shader);
-  glAttachShader(program_id, fragment_shader);
+  if (vertex_shader != 0)
+  {
+    glAttachShader(program_id, vertex_shader);
+  }
+  if (fragment_shader != 0)
+  {
+    glAttachShader(program_id, fragment_shader);
+  }
   if (geometry_shader != 0)
   {
     glAttachShader(program_id, geometry_shader);