vk: rt: tune emissive/additive blending so that it matches original

To match original more closely it additive geometry should be added to
final color in sRGB-γ space. You might not like that it is physically
incorrect, but this is what peak compatibility looks like.

Related to #668

ref/vk/NOTES.md

@@ -1079,3 +1079,11 @@ xash   vk (remapped)
 +Y = -X
 +X = +Z
 +Z = +Y
+
+# 2023-12-22 E352
+## sRGB vs γ blending
+Original:
+    `color = a + b`
+Our:
+    `color = sqrt(a*a + b*b)`
+There's nothing we can to do `a` only that would make it fake the "original" mixing result.

ref/vk/shaders/denoiser.comp

@@ -306,8 +306,18 @@ void main() {
 		colour = diffuse + specular;
 	}
 
+// See issue https://github.com/w23/xash3d-fwgs/issues/668, map test_blendmode_additive_alpha.
+// This macro enabled adding emissive to the final color in the *incorrect* sRGB-γ space. But it makes
+// it look much more like the original. Adding emissive in the *correct* linear space differs
+// from the original a lot, and looks perceptively worse.
+#define ADD_EMISSIVE_IN_GAMMA_SPACE
+#ifndef ADD_EMISSIVE_IN_GAMMA_SPACE
+	// Physically correct, but looks dull
 	colour += imageLoad(emissive, pix).rgb;
 	colour = LINEARtoSRGB(colour);
+#else // INCORRECT
+	colour = LINEARtoSRGB(colour) + LINEARtoSRGB(imageLoad(emissive, pix).rgb);
+#endif
 
 	imageStore(out_dest, pix, vec4(colour, 0./*unused*/));
 }