I'm using Mesa 19.1-git snapshots. I'll try again after the next update and inform you about my results.
We're working toward a new release in the next few days and after that I'll dig a little deeper and see if there is any work around I can put in the engine. The new release does include an updated BGFX (graphics) library so it might just magically be fixed but I'll do some more digging. Thanks for being patient with us.
I'm getting this in Ubuntu as well when I install the open source drivers for my Radeon card. Two things that worry me is that the 19.1 .0 release of Mesa is flagged as a "development release" and package maintainers seem to be building it without NDEBUG defined (so asserts can be triggered). I'm not sure there is anything I can so about this I'm afraid, can you roll back to the previous version as the release notes mention? https://www.mesa3d.org/relnotes/19.1.0.html
oh dear, this is a proper mess isn't it? I'm sorry it's not working for you. My Xubuntu is on Mesa 19.1.0-devel, LLVM 7.0.1 and I'm getting the assert as well. Annoyingly my Radeon R9 380 isn't fully supported by Gallium3D and I only get a few limited resolutions which is making this hard to debug (I usually use the amd-gpu proprietary drivers). I'll do some more digging today and maybe get in contact with the BGFX developers to see if they have an hints on a work around.
Thanks for your patience with this. Do you happen to remember which version of Mesa etc. you used when it was working?
I've reported a bug to FreeDesktop.org, hopefully this will be resolved soon: https://bugs.freedesktop.org/show_bug.cgi?id=109754 I'm nit 100% sure if it's a radeon_dri bug or a packaging bug but we'll get to the bottom of it. Thanks for your patience.