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.