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_vertex_buffer.hh

@@ -10,7 +10,9 @@
 
 #include "BLI_compiler_attrs.h"
 
-struct GPUVertBuf;
+namespace blender::gpu {
+class VertBuf;
+}
 
 extern PyTypeObject BPyGPUVertBuf_Type;
 
@@ -19,7 +21,7 @@ extern PyTypeObject BPyGPUVertBuf_Type;
 struct BPyGPUVertBuf {
   PyObject_VAR_HEAD
   /* The buf is owned, we may support thin wrapped batches later. */
-  GPUVertBuf *buf;
+  blender::gpu::VertBuf *buf;
 };
 
-PyObject *BPyGPUVertBuf_CreatePyObject(GPUVertBuf *buf) ATTR_NONNULL(1);
+PyObject *BPyGPUVertBuf_CreatePyObject(blender::gpu::VertBuf *buf) ATTR_NONNULL(1);