On Mesa >= 18.0.0, glXChooseFBConfig selects better GLCapabilities FBConfig than actually supported by glXCreatePbuffer and glXCreateGLXPixmap.
As tested on Mesa 18.3.6, requesting an RGB 8bit color component FBConfig for GLX_PBUFFER_BIT and GLX_PIXMAP_BIT GLX_DRAWABLE_TYPEs via glXChooseFBConfig returns an RGB 10bit color component FBConfig as its best match. Subsequent glXCreatePbuffer and glXCreateGLXPixmap calls fail.
This bugs seems to occur in Mesa >= 18.0.0 using allow_rgb10_configs, which is the default now. While the 10 bit color components are not listed for on-screen GLX.GLX_WINDOW_BIT GLX_DRAWABLE_TYPEs, they are listed for above mentioned off-screen types without XVisualInfo reference.
This quirk disables using any color component > 8 bit for GLX_PBUFFER_BIT and GLX_PIXMAP_BIT types and forces using an optional given GLCapabilitiesChooser or the DefaultGLCapabilitiesChooser.
Note: Also implies DontChooseFBConfigBestMatch for GLX_PBUFFER_BIT and GLX_PIXMAP_BIT types and hence depends on Bug 1391
Implemented as described