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

doc/python_api/sphinx_doc_gen.py

@@ -235,7 +235,9 @@ else:
         "bpy.utils.previews",
         "bpy_extras",
         "gpu",
-        "gpu.offscreen",
+        "gpu.types",
+        "gpu.matrix",
+        "gpu.select",
         "idprop.types",
         "mathutils",
         "mathutils.bvhtree",
@@ -1822,7 +1824,10 @@ def write_rst_importable_modules(basepath):
         # C_modules
         "aud": "Audio System",
         "blf": "Font Drawing",
-        "gpu.offscreen": "GPU Off-Screen Buffer",
+        "gpu": "GPU Shader Module",
+        "gpu.types": "GPU Types",
+        "gpu.matrix": "GPU Matrix",
+        "gpu.select": "GPU Select",
         "bmesh": "BMesh Module",
         "bmesh.types": "BMesh Types",
         "bmesh.utils": "BMesh Utilities",
@@ -1865,7 +1870,6 @@ def copy_handwritten_rsts(basepath):
     # TODO put this docs in Blender's code and use import as per modules above
     handwritten_modules = [
         "bgl",  # "Blender OpenGl wrapper"
-        "gpu",  # "GPU Shader Module"
         "bmesh.ops",  # generated by rst_from_bmesh_opdefines.py
 
         # includes...