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_offscreen.c

@@ -53,6 +53,8 @@
 #include "../generic/py_capi_utils.h"
 
 #include "gpu_py.h"
+#include "gpu_py_texture.h"
+
 #include "gpu_py_offscreen.h" /* own include */
 
 /* Define the free method to avoid breakage. */
@@ -264,6 +266,18 @@ static PyObject *pygpu_offscreen_color_texture_get(BPyGPUOffScreen *self, void *
   return PyLong_FromLong(GPU_texture_opengl_bindcode(texture));
 }
 
+PyDoc_STRVAR(pygpu_offscreen_texture_color_doc,
+             "The color texture attached.\n"
+             "\n"
+             ":type: :class:`gpu.types.GPUTexture`");
+static PyObject *pygpu_offscreen_texture_color_get(BPyGPUOffScreen *self, void *UNUSED(type))
+{
+  BPY_GPU_OFFSCREEN_CHECK_OBJ(self);
+  GPUTexture *texture = GPU_offscreen_color_texture(self->ofs);
+  GPU_texture_ref(texture);
+  return BPyGPUTexture_CreatePyObject(texture);
+}
+
 PyDoc_STRVAR(
     pygpu_offscreen_draw_view3d_doc,
     ".. method:: draw_view3d(scene, view_layer, view3d, region, view_matrix, projection_matrix)\n"
@@ -385,6 +399,11 @@ static PyGetSetDef pygpu_offscreen__tp_getseters[] = {
      (setter)NULL,
      pygpu_offscreen_color_texture_doc,
      NULL},
+    {"texture_color",
+     (getter)pygpu_offscreen_texture_color_get,
+     (setter)NULL,
+     pygpu_offscreen_texture_color_doc,
+     NULL},
     {"width", (getter)pygpu_offscreen_width_get, (setter)NULL, pygpu_offscreen_width_doc, NULL},
     {"height", (getter)pygpu_offscreen_height_get, (setter)NULL, pygpu_offscreen_height_doc, NULL},
     {NULL, NULL, NULL, NULL, NULL} /* Sentinel */