Refactor: Remove unnecessary C wrappers for vertex and index buffers

Now that all relevant code is C++, the indirection from the C struct
`GPUVertBuf` to the C++ `blender::gpu::VertBuf` class just adds
complexity and necessitates a wrapper API, making more cleanups like
use of RAII or other C++ types more difficult.

This commit replaces the C wrapper structs with direct use of the
vertex and index buffer base classes. In C++ we can choose which parts
of a class are private, so we don't risk exposing too many
implementation details here.

Pull Request: https://projects.blender.org/blender/blender/pulls/119825

source/blender/python/gpu/gpu_py_element.cc

@@ -246,7 +246,7 @@ PyTypeObject BPyGPUIndexBuf_Type = {
 /** \name Public API
  * \{ */
 
-PyObject *BPyGPUIndexBuf_CreatePyObject(GPUIndexBuf *elem)
+PyObject *BPyGPUIndexBuf_CreatePyObject(blender::gpu::IndexBuf *elem)
 {
   BPyGPUIndexBuf *self;