Python GPU: Add new methods to port the code templates in the manual

This commit adds a new API tha allow to replace the bgl API in the exemples on:
https://docs.blender.org/api/current/gpu.html

**Overview (New API):**
```
gpu.state:      active_framebuffer_get
GPUFramebuffer: read_color
GPUOffscreen:   texture_color
```

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D11031

source/blender/python/gpu/gpu_py_state.c

@@ -25,11 +25,13 @@
 
 #include <Python.h>
 
+#include "GPU_framebuffer.h"
 #include "GPU_state.h"
 
 #include "../generic/py_capi_utils.h"
 #include "../generic/python_utildefines.h"
 
+#include "gpu_py_framebuffer.h"
 #include "gpu_py_state.h" /* own include */
 
 /* -------------------------------------------------------------------- */
@@ -334,6 +336,16 @@ static PyObject *pygpu_state_program_point_size_set(PyObject *UNUSED(self), PyOb
   Py_RETURN_NONE;
 }
 
+PyDoc_STRVAR(pygpu_state_framebuffer_active_get_doc,
+             ".. function:: framebuffer_active_get(enable)\n"
+             "\n"
+             "   Return the active framefuffer in context.\n");
+static PyObject *pygpu_state_framebuffer_active_get(PyObject *UNUSED(self))
+{
+  GPUFrameBuffer *fb = GPU_framebuffer_active_get();
+  return BPyGPUFrameBuffer_CreatePyObject(fb, true);
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */
@@ -396,6 +408,10 @@ static struct PyMethodDef pygpu_state__tp_methods[] = {
      (PyCFunction)pygpu_state_program_point_size_set,
      METH_O,
      pygpu_state_program_point_size_set_doc},
+    {"active_framebuffer_get",
+     (PyCFunction)pygpu_state_framebuffer_active_get,
+     METH_NOARGS,
+     pygpu_state_framebuffer_active_get_doc},
     {NULL, NULL, 0, NULL},
 };