layers:Separate state update from addCmd()

layers:Separate state update from addCmd()

Update addCmd() function to only perform validation and rename it to
ValidateCmd(). Separate out the state update of last_cmd in cmd buffer
to new UpdateCmdBufferLastCmd() function.

Made sure that everywhere ValidateCmd() is called now also calls
UpdateCmdBufferLastCmd(). Currently just calling them back-to-back but
will continue with further updates that put the ValidateCmd() call into
the PreCallValidate*() function and the UpdateCmdBufferLastCmd() into
the PostCallRecord*() function.

Also included some comments on work that needs to be done while
breaking up CmdDraw() for pre/post functions.