So, there appear to be two problems here, a surface one and an underlying one.
The surface one is that EGL clients are failing, because for some reason the software rasteriser is failing to work against Mir.
The underlying one is that accelerated GL isn’t working at all, and we’re falling back to the software rasteriser. We appear to have the relevant kernel module and DRI driver built and installed, but it is failing to load. I will continue to investigate this.