mesa-kms fails to initialise on Cosmic

[RAOF]

The mesa-kms platform fails to initialise on 18.10 for me. The relevant error is:

ERROR: /build/mir-Bn3LCJ/mir-0.32.0~rc10-g29f51c4b18/src/platforms/mesa/server/kms/egl_helper.cpp(115): Throw in function void mir::graphics::mesa::helpers::EGLHelper::setup(const mir::graphics::mesa::helpers::GBMHelper&, gbm_surface*, EGLContext)
Dynamic exception type:  boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<std::system_error> >
std::exception::what: Failed to create EGL window surface: EGL_BAD_MATCH (0x3009)

[RAOF]

Oh, ho! It seems the problem is that Mesa now supports XRGB2101010 as a render target, and as eglChooseConfig selects the largest colour buffer size greater than or equal to what you request, we're now getting it. This then fails to match with the XRGB8888 gbm_surface we're trying to render to…

[RAOF]

So what we probably should do here is what Weston does - rather than ask for a EGLConfig with some bits set and hope that it'll match our GBM format, do a more thorough config selection and explicitly match the GBM format with the visual_id, guaranteeing we get either no matching config or a config that actually matches the GBM format we're using…