Fix T42401: Gaussian blur node is visibly squared-off at larger sizes

The root of the issue comes to the way how we sample the gaussian filter in
RE_filter_value(). We need to scale x to -3*sigma..3*sigma segment in order
to get the whole bell.

The old code tried to do it, but failed dramatically, plus it used some weird
gaussian sampling formula. Replaced it with much more clear one, which gives
proper blur now.

There's no visible different in AA sampling in BI render tho.

Other filters like Mitchell still tends to give wrong square shaped blurs,
but they're much more difficult to resolve because they're just wrong in
the code -- for some reason smaller kernel size means more blur. Let's solve
this later.

source/blender/render/intern/source/initrender.c

@@ -164,8 +164,11 @@ float RE_filter_value(int type, float x)
 			return 1.0f - x;
 			
 		case R_FILTER_GAUSS:
-			x *= gaussfac;
-			return (1.0f / expf(x * x) - 1.0f / expf(gaussfac * gaussfac * 2.25f));
+		{
+			const float two_gaussfac2 = 2.0f * gaussfac * gaussfac;
+			x *= 3.0f * gaussfac;
+			return 1.0f / sqrtf(M_PI * two_gaussfac2) * expf(-x*x / two_gaussfac2);
+		}
 			
 		case R_FILTER_MITCH:
 			return filt_mitchell(x * gaussfac);