Bindings additions, miscellaneous changes

Working on scene bindings. Moved GRID_CONSTANT macro into
bindnigs/common.h and renamed it to BIND_GLOBAL_CONSTANT. Used by multiple
constants registering functions.

Added get_sub_count() scene_node API call. Get number of subordinate nodes
to a given node. Value is cached/updated if it is "dirty". To get this
value from a constant scene_node, just use cached value .sub_count
instead.

Add version definitions to hydra.h; open Lua libraries in engine/main.c
initialization. Important note: must only open required libs, i.e. math,
string, table libs (perhaps others). Things like OS, I/O libs are not
needed; that functionality will be supplied through engine bindings so
we can enforce what operations scripts are permitted to perform at OS
and filesystem level.