Implement GL_EXT_base_instance Added new extension GL_EXT_base_instance to registry_xml.py and auto-generated the entry point function calls for the following functions introduced by the extension: * GL_DrawElementsInstancedBaseVertexBaseInstanceEXT * GL_DrawElementsInstancedBaseInstanceEXT * GL_DrawArraysInstancedBaseInstanceEXT Bug: angleproject:6983 Change-Id: I36167faf3ca98e42acf787dbf09ee7052e15e358 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3691952 Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Commit-Queue: Faye Zhang <ffz@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com>