Show OpenCL VRAM usage in statistics

[Theverat]

Old LuxBlend code:

# Memory usage (only available for OpenCL engines)
if str.endswith(engine, 'OCL') and rendering_controls.stats_memory:
    device_stats = stats.Get("stats.renderengine.devices")

    max_memory = float('inf')
    used_memory = 0

    for i in range(device_stats.GetSize()):
        device_name = device_stats.GetString(i)

        # Max memory available is limited by the device with least amount of memory
        device_max_memory = stats.Get("stats.renderengine.devices." + device_name + ".memory.total").GetFloat()
        device_max_memory = int(device_max_memory / (1024 * 1024))
        if device_max_memory < max_memory:
            max_memory = device_max_memory

        device_used_memory = stats.Get("stats.renderengine.devices." + device_name + ".memory.used").GetFloat()
        device_used_memory = int(device_used_memory / (1024 * 1024))
        if device_used_memory > used_memory:
            used_memory = device_used_memory

    if used_memory > self.mem_peak:
        self.mem_peak = used_memory

    stats_list.append('Memory: %d MB/%d MB' % (used_memory, max_memory))

When implementing this, check if stats.renderengine.devices really only returns the enabled devices (important for max. available memory calculation).