Added support for alpha blending using palette alpha

Fixes https://github.com/libsdl-org/SDL_image/issues/409

src/video/SDL_blit_1.c

@@ -435,7 +435,7 @@ static void Blit1toNAlpha(SDL_BlitInfo *info)
     const SDL_Color *srcpal = info->src_fmt->palette->colors;
     int dstbpp;
     Uint32 pixel;
-    unsigned sR, sG, sB;
+    unsigned sR, sG, sB, sA;
     unsigned dR, dG, dB, dA;
     const unsigned A = info->a;
 
@@ -449,8 +449,9 @@ static void Blit1toNAlpha(SDL_BlitInfo *info)
             sR = srcpal[*src].r;
             sG = srcpal[*src].g;
             sB = srcpal[*src].b;
+            sA = (srcpal[*src].a * A) / 255;
             DISEMBLE_RGBA(dst, dstbpp, dstfmt, pixel, dR, dG, dB, dA);
-            ALPHA_BLEND_RGBA(sR, sG, sB, A, dR, dG, dB, dA);
+            ALPHA_BLEND_RGBA(sR, sG, sB, sA, dR, dG, dB, dA);
             ASSEMBLE_RGBA(dst, dstbpp, dstfmt, dR, dG, dB, dA);
             src++;
             dst += dstbpp;
@@ -475,7 +476,7 @@ static void Blit1toNAlphaKey(SDL_BlitInfo *info)
     Uint32 ckey = info->colorkey;
     int dstbpp;
     Uint32 pixel;
-    unsigned sR, sG, sB;
+    unsigned sR, sG, sB, sA;
     unsigned dR, dG, dB, dA;
     const unsigned A = info->a;
 
@@ -490,8 +491,9 @@ static void Blit1toNAlphaKey(SDL_BlitInfo *info)
                 sR = srcpal[*src].r;
                 sG = srcpal[*src].g;
                 sB = srcpal[*src].b;
+                sA = (srcpal[*src].a * A) / 255;
                 DISEMBLE_RGBA(dst, dstbpp, dstfmt, pixel, dR, dG, dB, dA);
-                ALPHA_BLEND_RGBA(sR, sG, sB, A, dR, dG, dB, dA);
+                ALPHA_BLEND_RGBA(sR, sG, sB, sA, dR, dG, dB, dA);
                 ASSEMBLE_RGBA(dst, dstbpp, dstfmt, dR, dG, dB, dA);
             }
             src++;
@@ -531,8 +533,10 @@ SDL_BlitFunc SDL_CalculateBlit1(SDL_Surface *surface)
         return one_blitkey[which];
 
     case SDL_COPY_COLORKEY | SDL_COPY_BLEND:  /* this is not super-robust but handles a specific case we found sdl12-compat. */
-        return (surface->map->info.a == 255) ? one_blitkey[which] : (SDL_BlitFunc)NULL;
+        return (surface->map->info.a == 255) ? one_blitkey[which] :
+                which >= 2 ? Blit1toNAlphaKey : (SDL_BlitFunc)NULL;
 
+    case SDL_COPY_BLEND:
     case SDL_COPY_MODULATE_ALPHA | SDL_COPY_BLEND:
         /* Supporting 8bpp->8bpp alpha is doable but requires lots of
            tables which consume space and takes time to precompute,