Bug fix #1947

Unified render added sky with alpha=1. Found the way how to fix it, didnt
dare that yesterday. :)
Now sky is rendered with alpha=0, like normal render.

Thanks to the recent reports, a lot of old issues with unified render have
been solved. This is going to be pretty OK!

source/blender/render/intern/source/pixelblending.c

@@ -399,21 +399,6 @@ void addalphaAddfacFloat(float *dest, float *source, char addfac)
     float m; /* weiging factor of destination */
     float c; /* intermediate colour           */
 
-    /* 1. copy source straight away if dest has zero alpha */
-	/* 2. copy dest straight away if dest has full alpha   */
-	/* I am not sure whether (2) is correct. It seems to   */
-	/* me that this should not happen if float colours     */
-	/* aren't clipped at 1.0 .                             */
-	/* I'll keep the code, but disabled....                */
-    if ( (dest[3] < RE_EMPTY_COLOUR_FLOAT) 
-		/*   || source[3] > RE_FULL_COLOUR_FLOAT */ ) {
-        dest[0] = source[0];
-        dest[1] = source[1];
-        dest[2] = source[2];
-        dest[3] = source[3];
-        return;
-    }
-
     /* Addfac is a number between 0 and 1: rescale */
     /* final target is to diminish the influence of dest when addfac rises */
     m = 1.0 - ( source[3] * ((255.0 - addfac) / 255.0));
@@ -884,12 +869,6 @@ void addalphaAddfacshort(unsigned short *doel,
 {
 	int c, mul;
 
-	if( doel[3]==0) {
-		*((unsigned int *)doel)= *((unsigned int *)bron);
-		*((unsigned int *)(doel+2))= *((unsigned int *)(bron+2));
-		return;
-	}
-
 	mul= 0xFFFF - (bron[0]*(255-addfac))/255;
 	
 	c= ((mul*doel[0])>>16)+bron[0];

source/blender/render/intern/source/pixelshading.c

@@ -418,6 +418,7 @@ void renderSkyPixelFloat(float x, float y)
 	case RE_ALPHA_SKY:
 		/* Fill in the sky as if it were a normal face. */
 		shadeSkyPixel(x, y);
+		collector[3]= 0.0;
 		break;
 	default:
 		; /* Error: illegal alpha blending state */