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/intern/CMakeLists.txt

@@ -82,10 +82,6 @@ set(SRC
 	bpy_traceback.c
 	bpy_utils_previews.c
 	bpy_utils_units.c
-	gpu.c
-	gpu_offscreen.c
-	gpu_py_matrix.c
-	gpu_py_select.c
 	stubs.c
 
 	bpy.h
@@ -120,7 +116,6 @@ set(SRC
 	bpy_traceback.h
 	bpy_utils_previews.h
 	bpy_utils_units.h
-	gpu.h
 	../BPY_extern.h
 	../BPY_extern_clog.h
 )