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.hh

@@ -38,6 +38,11 @@ void update_bindings(const GLuint vao,
                      const ShaderInterface *interface,
                      const int base_instance);
 
+void update_bindings(const GLuint vao,
+                     const uint v_first,
+                     const GPUVertFormat *format,
+                     const ShaderInterface *interface);
+
 }  // namespace GLVertArray
 
 }  // namespace gpu