Optimize string copying in rebuffer() and stgwrite()

[Daniel-Cortez]

In this patch I've replaced loops with CHECK_STGPIPE/CHECK_STGBUFFER checks at every iteration with a single buffer growth check and a memcpy call.
This doesn't do much performance-wise, but I think it's still good to have.

Also it seems that the same kind of optimisation has already been applied in stgwrite within the else branch, but for some reason not within the if (staging) branch.

[maddinat0r]

Also it seems that the same kind of optimisation has already been applied in stgwrite within the else branch, but for some reason not within the if (staging) branch.

That was actually my doing! The unoptimized code in the else branch was overall a pretty big bottleneck back then, that's why I optimized only that part.