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/examples/gpu.types.GPUOffScreen.py

@@ -42,7 +42,7 @@ class OffScreenDraw(bpy.types.Operator):
         aspect_ratio = scene.render.resolution_x / scene.render.resolution_y
 
         try:
-            offscreen = gpu.offscreen.new(512, int(512 / aspect_ratio))
+            offscreen = gpu.types.GPUOffScreen(512, int(512 / aspect_ratio))
         except Exception as e:
             print(e)
             offscreen = None
@@ -52,7 +52,7 @@ class OffScreenDraw(bpy.types.Operator):
     @staticmethod
     def _update_offscreen(context, offscreen):
         scene = context.scene
-        render_layer = context.render_layer
+        view_layer = context.view_layer
         render = scene.render
         camera = scene.camera
 

doc/python_api/rst/gpu.rst

@@ -1,20 +0,0 @@
-*******************
-GPU functions (gpu)
-*******************
-
-.. module:: gpu
-
-Functions for GPU offscreen rendering, matrix stacks and selection.
-
-Submodules:
-
-.. toctree::
-   :maxdepth: 1
-
-   gpu.offscreen.rst
-
-
-Intro
-=====
-
-Module to provide functions concerning the GPU implementation in Blender.

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...