Working on bug 3921 - Add some Fastpath to BlitNtoNKey and BlitNtoNKeyCopyAlpha

Sylvain

I did various benches. with clang 6.0.0 on linux, and ndk-r16b on android (NDK_TOOLCHAIN_VERSION=clang).

- still see a x10 speed factor.
- with duff_loops, it does not use vectorisation (but doesn't seem to be a problem).

on linux my patch is already at full speed on -O2, whereas the duff_loops need -O3 (200 ms at -03, and 300ms at -02).

I realized that on Android, I had a slight variation which fits best.
both on linux with -O2 and -O3, and on android with 02/03 and armeabi-v7a/arm64.

Here's the patch.

src/video/SDL_blit_N.c

@@ -2344,8 +2344,9 @@ BlitNtoNKey(SDL_BlitInfo * info)
             /* *INDENT-OFF* */
             DUFFS_LOOP(
             {
-                Uint32 Pixel = (*src32 == ckey) ? *dst32 : *src32;
-                *dst32 = Pixel;
+                if (*src32 != ckey) {
+                    *dst32 = *src32;
+                }
                 ++src32;
                 ++dst32;
             },
@@ -2418,8 +2419,9 @@ BlitNtoNKeyCopyAlpha(SDL_BlitInfo * info)
             /* *INDENT-OFF* */
             DUFFS_LOOP(
             {
-                Uint32 Pixel_ = ((*src32 & rgbmask) == ckey) ? *dst32 : *src32;
-                *dst32 = Pixel_;
+                if ((*src32 & rgbmask) != ckey) {
+                    *dst32 = *src32;
+                }
                 ++src32;
                 ++dst32;
             },