GPUIndexBuf: Find the minimum and maximum index through the builder

Moving the bounds code to the builder can be useful
for future optimizations like building multithreaded.

Reviewed By: fclem, jbakker

Differential Revision: https://developer.blender.org/D11455

source/blender/gpu/intern/gpu_index_buffer_private.hh

@@ -73,7 +73,7 @@ class IndexBuf {
   IndexBuf(){};
   virtual ~IndexBuf();
 
-  void init(uint indices_len, uint32_t *indices);
+  void init(uint indices_len, uint32_t *indices, uint min_index, uint max_index);
   void init_subrange(IndexBuf *elem_src, uint start, uint length);
   void init_build_on_device(uint index_len);