mesa: delay copying of debug groups

mesa: delay copying of debug groups

Do not copy the debug group until it is about to be written.  One likely
scenario of using glPushDebugGroup/glPopDebugGroup is to enclose a sequence of
GL commands and give them a human-readable description.  There is no message
control change in this scenario, and thus no need to copy.

This also reduces the initial size of gl_debug_state from 306KB to 7KB.

Signed-off-by: Chia-I Wu <olv@lunarg.com>
Reviewed-by: Brian Paul <brianp@vmware.com>