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/bpy_interface.c

@@ -46,7 +46,6 @@
 #include "RNA_types.h"
 
 #include "bpy.h"
-#include "gpu.h"
 #include "bpy_rna.h"
 #include "bpy_path.h"
 #include "bpy_capi_utils.h"
@@ -219,7 +218,6 @@ static struct _inittab bpy_internal_modules[] = {
 	{"mathutils.kdtree", PyInit_mathutils_kdtree},
 #endif
 	{"_bpy_path", BPyInit__bpy_path},
-	{"_gpu", BPyInit_gpu},
 	{"bgl", BPyInit_bgl},
 	{"blf", BPyInit_blf},
 	{"imbuf", BPyInit_imbuf},
@@ -235,7 +233,7 @@ static struct _inittab bpy_internal_modules[] = {
 #ifdef WITH_CYCLES
 	{"_cycles", CCL_initPython},
 #endif
-	{"gpu", GPU_initPython},
+	{"gpu", BPyInit_gpu},
 	{"idprop", BPyInit_idprop},
 	{NULL, NULL}
 };