GPU: Cleanup implementation casts

- Use the syntactic wrap/unwrap method to make code more readable.
- Update comment about hidden struct behind opaque types.
- Cleanup GPUDrawList type.

source/blender/gpu/GPU_index_buffer.h

@@ -31,11 +31,7 @@
 extern "C" {
 #endif
 
-/**
- * IMPORTANT: Do not allocate manually as the real struct is bigger (i.e: GLIndexBuf). This is only
- * the common and "public" part of the struct. Use the provided allocator.
- * TODO(fclem) Make the content of this struct hidden and expose getters/setters.
- **/
+/** Opaque type hiding blender::gpu::IndexBuf. */
 typedef struct GPUIndexBuf GPUIndexBuf;
 
 GPUIndexBuf *GPU_indexbuf_calloc(void);