util/log: add log level fatal and log level env
Required for !17666
Adds a MESA_LOG_FATAL
logging level.
Adds a MESA_LOG_LEVEL
env that sets the minimum logging level to common logging that is very similar to EGL_LOG_LEVEL
.
FALLBACK_LOG_LEVEL
shall be at least MESA_LOG_WARN
, otherwise, the "invalid logging level" message is not going to be shown.