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_vertex_buffer.h

@@ -0,0 +1,42 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file blender/python/gpu/gpu_py_vertex_buffer.h
+ *  \ingroup bpygpu
+ */
+
+#ifndef __GPU_PY_VERTEX_BUFFER_H__
+#define __GPU_PY_VERTEX_BUFFER_H__
+
+#include "BLI_compiler_attrs.h"
+
+extern PyTypeObject BPyGPUVertBuf_Type;
+
+#define BPyGPUVertBuf_Check(v)        (Py_TYPE(v) == &BPyGPUVertBuf_Type)
+
+typedef struct BPyGPUVertBuf {
+	PyObject_VAR_HEAD
+	/* The buf is owned, we may support thin wrapped batches later. */
+	struct GPUVertBuf *buf;
+} BPyGPUVertBuf;
+
+PyObject *BPyGPUVertBuf_CreatePyObject(struct GPUVertBuf *vbo) ATTR_NONNULL(1);
+
+#endif /* __GPU_PY_VERTEX_BUFFER_H__ */