Proper implementation of crossfading effect

Now it looks and feels like vanilla!

src/i_video.c

@@ -2187,6 +2187,16 @@ const pixel_t I_BlendOverAltXlatab (const pixel_t bg, const pixel_t fg)
 	return amask | r | g | b;
 }
 
+// [crispy] Perform screen crossfading effect by given amount of opacity, used for Strife
+const pixel_t I_BlendOverCrossfade (const pixel_t bg, const pixel_t fg, const int amount)
+{
+	const uint32_t r = ((amount * (fg & rmask) + (0xff - amount) * (bg & rmask)) >> 8) & rmask;
+	const uint32_t g = ((amount * (fg & gmask) + (0xff - amount) * (bg & gmask)) >> 8) & gmask;
+	const uint32_t b = ((amount * (fg & bmask) + (0xff - amount) * (bg & bmask)) >> 8) & bmask;
+
+	return amask | r | g | b;
+}
+
 const pixel_t (*blendfunc) (const pixel_t fg, const pixel_t bg) = I_BlendOver;
 
 const pixel_t I_MapRGB (const uint8_t r, const uint8_t g, const uint8_t b)

src/strife/f_wipe.c

@@ -109,7 +109,9 @@ wipe_doColorXForm
 #ifndef CRISPY_TRUECOLOR
             *cur_screen = xlatab[(*cur_screen << 8) + *end_screen];
 #else
-            *cur_screen = I_BlendOverXlatab(*cur_screen, *end_screen);
+            // [crispy] perform crossfading effect with 13 given opacity steps, multipled by 19:
+            // 247, 228, 209, 190, 171, 152, 133, 114, 95, 76, 57, 38, 19
+            *cur_screen = I_BlendOverCrossfade(*end_screen, *cur_screen, fade_safe_tics * 19);
 #endif
         }
         ++cur_screen;

src/v_trans.h

@@ -67,6 +67,7 @@ extern const pixel_t I_BlendOverTinttab (const pixel_t bg, const pixel_t fg);
 extern const pixel_t I_BlendOverAltTinttab (const pixel_t bg, const pixel_t fg);
 extern const pixel_t I_BlendOverXlatab (const pixel_t bg, const pixel_t fg);
 extern const pixel_t I_BlendOverAltXlatab (const pixel_t bg, const pixel_t fg);
+extern const pixel_t I_BlendOverCrossfade (const pixel_t bg, const pixel_t fg, const int amount);
 #endif
 
 int V_GetPaletteIndex(byte *palette, int r, int g, int b);