[sldev] lltexturefetch.cpp patch for http-texture and/or snowglobe

[Steve Bennetts]

We will be applying this patch next week, but if someone wants to play, 
I just discovered this major bug in lltexturefetch.cpp:

void LLTextureFetchWorker::callbackDecoded(bool success, LLImageRaw* 
raw, LLImageRaw* aux)
{
+    LLMutexLock lock(&mWorkMutex);

    ...


The missing mutex lock here can cause all sorts of terrible artifacts. 
Don't know if this will fix everything, but it definitely addresses at 
least one of the crashes I have been seeing.

-Steve