Mesa removed that library in 027ccd963b1f8f288bef4224aedcddc1557e4f8a,
so this API can be removed from GLFW too.
Mesa has provided an alternative path for years, through the
EGL_MESA_platform_surfaceless EGL extension, and it also supports
selecting the software rasterizer through the EGL_EXT_platform_device
EGL extension, neither of which are supported by GLFW yet.
I kept the external API the same, notably the GLFW_OSMESA_CONTEXT_API
define is still present, but will error at runtime.
c27caa4b1d
