Graphics glitches in VirtualBox Ubuntu guest when 3D acceleration is enabled

I am using Ubuntu 16.04 as a VirtualBox guest on a Windows 10 host machine, I've installed and re-installed guest additions by following the instruction on this page. The drivers appear to be configured fine. /usr/lib/nux/unity_support_test -p gives:

OpenGL vendor string:   Humper
OpenGL renderer string: Chromium
OpenGL version string:  2.1 Chromium 1.9

Not software rendered:    yes
Not blacklisted:          yes
GLX fbconfig:             yes
GLX texture from pixmap:  yes
GL npot or rect textures: yes
GL vertex program:        yes
GL fragment program:      yes
GL vertex buffer object:  yes
GL framebuffer object:    yes
GL version is 1.4+:       yes

Unity 3D supported:       yes

Screenshots of some of the glitches:

How can I fix this?