make the maximum render batch size a constant
make the maximum render batch size a constant

Summary: To make the render batch size stand out more, make it a constant at the top of the file.

Reviewed By: wez

Differential Revision: D21414166

fbshipit-source-id: 1a7e913e689961909098a6bc9b6f265adbb3a7ff

facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman