Mesa: allow suppression of debug messages in a debug build

Mesa: allow suppression of debug messages in a debug build

For testing, it's very useful to be able to test on a debug build,
while suppressing the debug messages (messages that are by default
suppressed in a release build), in order to see the same behavior
that users of release builds will see.

For example, the "piglit" test suite will flag an error on
programs that produce unexpected output, which means that a
debug build will always fail due to the extra debug messages.

This change introduces a new value to the MESA_DEBUG
environment variable.  In a debug build, explicitly setting MESA_DEBUG
to "0" will suppress all debug messages (both from _mesa_debug() and
from _mesa_warning()).  (The former behavior was that debug
messages were never suppressed in debug builds.)

Behavior of non-debug builds has not changed.  In such a build,
_mesa_debug() messages are always suppressed, and _mesa_warning()
messages will be suppressed unless MESA_DEBUG is set *to any value*.