mesa: rework the meaning of gl_debug_message::length

mesa: rework the meaning of gl_debug_message::length

Currently it stores strlen(buf) whenever the user originally provided a
negative value for length.

Although I've not seen any explicit text in the spec, CTS requires that
the very same length (be that negative value or not) is returned back on
Pop.

So let's push down the length < 0 checks, tweak the meaning of
gl_debug_message::length and fix GetDebugMessageLog to add and count the
null terminators, as required by the spec.

v2: return correct total length in GetDebugMessageLog
v3: rebase (drop _mesa_shader_debug hunk).

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>