feat: add job system extension

[hoffstadt]

Purpose

The purpose of this pull request is to add an extension that implements a simple but flexible job system. This is an extension built on top of the recently introduced threads API. This extension is ready for 1.0 status (API is stable).

Design

The design is relatively simple. Users submit an array of jobs and receive an atomic counter. This counter's initial value is the number of jobs just submitted. Users later can wait on this counter to reach a specified value.

The atomic counters are managed through a free list (protected by the "latch" mentioned below).

A queue of jobs is protected by an atomic "latch". Threads remove jobs from the queue, execute them, then decrement their associated counter. If there are no jobs, the threads sleep on a conditional variable. When new jobs are pushed, threads are woken up using the conditional variable.

API

// job description
typedef struct _plJobDesc
{
    void (*task)(void* pData);
    void* pData;
} plJobDesc;

// API
typedef struct _plJobI
{
    // setup/shutdown
    void (*initialize)(uint32_t uThreadCount);
    void (*cleanup)(void);

    // typical usage
    void (*run_jobs)        (plJobDesc* ptJobs, uint32_t uJobCount, plAtomicCounter** pptCounter);
    void (*wait_for_counter)(plAtomicCounter* ptCounter, uint32_t uValue);
} plJobI;