GPUImmediate: GL backend isolation

This is part of the Vulkan backend task T68990.

This is mostly a cleanup, however, there is a small change:
We don't use a special Vertex Array binding function for Immediate
anymore and just reuse the one for batches.
This might create a bit more state changes but this could be fixed
easily if it causes perf regression.

# Conflicts:
#	source/blender/gpu/intern/gpu_context.cc

source/blender/gpu/opengl/gl_vertex_array.cc

@@ -155,6 +155,17 @@ void GLVertArray::update_bindings(const GLuint vao,
   }
 }
 
+/* Another version of update_bindings for Immediate mode. */
+void GLVertArray::update_bindings(const GLuint vao,
+                                  const uint v_first,
+                                  const GPUVertFormat *format,
+                                  const ShaderInterface *interface)
+{
+  glBindVertexArray(vao);
+
+  vbo_bind(interface, format, v_first, 0, false);
+}
+
 /** \} */
 
 }  // namespace blender::gpu