stb_image: Small PNG filter simplification

Paeth on the first scanline is not a separate filter, it
happens to be equivalent to the "sub" filter which is already
an option.

stb_image.h

@@ -4639,9 +4639,8 @@ enum {
    STBI__F_up=2,
    STBI__F_avg=3,
    STBI__F_paeth=4,
-   // synthetic filters used for first scanline to avoid needing a dummy row of 0s
-   STBI__F_avg_first,
-   STBI__F_paeth_first
+   // synthetic filter used for first scanline to avoid needing a dummy row of 0s
+   STBI__F_avg_first
 };
 
 static stbi_uc first_row_filter[5] =
@@ -4650,7 +4649,7 @@ static stbi_uc first_row_filter[5] =
    STBI__F_sub,
    STBI__F_none,
    STBI__F_avg_first,
-   STBI__F_paeth_first
+   STBI__F_sub // Paeth with b=c=0 turns out to be equivalent to sub
 };
 
 static int stbi__paeth(int a, int b, int c)
@@ -4719,9 +4718,8 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
             case STBI__F_sub        : cur[k] = raw[k]; break;
             case STBI__F_up         : cur[k] = STBI__BYTECAST(raw[k] + prior[k]); break;
             case STBI__F_avg        : cur[k] = STBI__BYTECAST(raw[k] + (prior[k]>>1)); break;
-            case STBI__F_paeth      : cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(0,prior[k],0)); break;
+            case STBI__F_paeth      : cur[k] = STBI__BYTECAST(raw[k] + prior[k]); break; // prior[k] == stbi__paeth(0,prior[k],0)
             case STBI__F_avg_first  : cur[k] = raw[k]; break;
-            case STBI__F_paeth_first: cur[k] = raw[k]; break;
          }
       }
 
@@ -4753,13 +4751,12 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
                 for (k=0; k < nk; ++k)
          switch (filter) {
             // "none" filter turns into a memcpy here; make that explicit.
-            case STBI__F_none:         memcpy(cur, raw, nk); break;
+            case STBI__F_none:               memcpy(cur, raw, nk); break;
             STBI__CASE(STBI__F_sub)          { cur[k] = STBI__BYTECAST(raw[k] + cur[k-filter_bytes]); } break;
             STBI__CASE(STBI__F_up)           { cur[k] = STBI__BYTECAST(raw[k] + prior[k]); } break;
             STBI__CASE(STBI__F_avg)          { cur[k] = STBI__BYTECAST(raw[k] + ((prior[k] + cur[k-filter_bytes])>>1)); } break;
             STBI__CASE(STBI__F_paeth)        { cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k-filter_bytes],prior[k],prior[k-filter_bytes])); } break;
             STBI__CASE(STBI__F_avg_first)    { cur[k] = STBI__BYTECAST(raw[k] + (cur[k-filter_bytes] >> 1)); } break;
-            STBI__CASE(STBI__F_paeth_first)  { cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k-filter_bytes],0,0)); } break;
          }
          #undef STBI__CASE
          raw += nk;
@@ -4776,7 +4773,6 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
             STBI__CASE(STBI__F_avg)          { cur[k] = STBI__BYTECAST(raw[k] + ((prior[k] + cur[k- output_bytes])>>1)); } break;
             STBI__CASE(STBI__F_paeth)        { cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k- output_bytes],prior[k],prior[k- output_bytes])); } break;
             STBI__CASE(STBI__F_avg_first)    { cur[k] = STBI__BYTECAST(raw[k] + (cur[k- output_bytes] >> 1)); } break;
-            STBI__CASE(STBI__F_paeth_first)  { cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k- output_bytes],0,0)); } break;
          }
          #undef STBI__CASE