Join the python modules gpu and _gpu into one.

Maybe it's still early to set the new drawing api for python.
But joining these two modules is an initial step.

```
>>> gpu.
        matrix
        select
        types
```
```
>>> gpu.types.GPU
                 Batch(
                 OffScreen(
                 VertBuf(
                 VertFormat(
```
The creation of a new offscreen object is now done by the `GPUOffscreen.__new__` method.

Reviewers: campbellbarton, dfelinto

Reviewed By: campbellbarton, dfelinto

Tags: #bf_blender_2.8

Differential Revision: https://developer.blender.org/D3667

source/blender/python/gpu/gpu_py_api.h

@@ -18,13 +18,13 @@
  * ***** END GPL LICENSE BLOCK *****
  */
 
+/** \file blender/python/gpu/gpu_py_api.h
+ *  \ingroup bpygpu
+ */
+
 #ifndef __GPU_PY_API_H__
 #define __GPU_PY_API_H__
 
-/** \file blender/python/gpu/gpu_py_api.h
- *  \ingroup pygpu
- */
-
 PyObject *BPyInit_gpu(void);
 
 #endif  /* __GPU_PY_API_H__ */