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