mesa: add MAX_PIXEL_BYTES define

mesa: add MAX_PIXEL_BYTES define

In a few places we need to allocate space for some number of generic
pixels.  Use this new define instead of a magic number like 16 or
4 * sizeof(GLuint).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>