[Graph]: reduce vertex buffer size in worker thread.
[Graph]: reduce vertex buffer size in worker thread.

It actually makes the program runs faster. Maybe smaller vertex buffer
can increase the CPU cache hits.

icoming/FlashGraph,icoming/FlashGraph,zheng-da/FlashX,flashxio/FlashX,icoming/FlashX,zheng-da/FlashX,flashxio/FlashX,icoming/FlashGraph,silky/FlashGraph,icoming/FlashX,flashxio/FlashX,zheng-da/FlashX,silky/FlashGraph,flashxio/FlashX,icoming/FlashX,icoming/FlashGraph,silky/FlashGraph,icoming/FlashX,flashxio/FlashX,zheng-da/FlashX,silky/FlashGraph,silky/FlashGraph,icoming/FlashX,icoming/FlashGraph,flashxio/FlashX,zheng-da/FlashX