mesa: reduce memory used for short display lists

mesa: reduce memory used for short display lists

Display lists allocate memory in chunks of 256 tokens (1KB) at a time.
If an app creates many short display lists or uses glXUseXFont() this
can waste quite a bit of memory.

This patch uses realloc() to trim short lists and reduce the memory
used.

Also, null/zero-out some list construction fields in _mesa_EndList().

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>