Use a better random number generator

[programagor]

Use of rand() is discouraged, because the quality of the produced distribution has some undesirable characteristics. I haven't noticed any effects of that in the images, but better play it safe.

rainbrot/src/worker.c

Lines 66 to 71 in baffa37

	do
	{
	U1=rand();
	}
	while(U1==0);
	U2=rand();