stb_image_write.h: Fix jpg flipping for non-multiple-of-8 sizes

JPG always encodes 8x8 pixel blocks. If the input image does not have
a width or height that's a multiple of 8, the last column or row is just
used multiple times for the remaining pixels of the block.
The original code first calculated p (the index into the pixel data)
with the "imaginary" row/colum (that might be up to 7 pixels too far
into each direction) and then subtracted the necessary amount of bytes
it if row >= height or col >= width.
That was a bit cryptic (IMHO), and didn't get more readable/obvious when
vertical flipping was added - which introduced a bug, by not taking
stbi__flip_vertically_on_write into account when adjusting p for
row >= height...

The code should be more obvious (and less buggy) now.

This fixes bug #592

stb_image_write.h

@@ -1424,14 +1424,16 @@ static int stbi_write_jpg_core(stbi__write_context *s, int width, int height, in
             float YDU[64], UDU[64], VDU[64];
             for(row = y, pos = 0; row < y+8; ++row) {
                for(col = x; col < x+8; ++col, ++pos) {
-                  int p = (stbi__flip_vertically_on_write ? height-1-row : row)*width*comp + col*comp;
                   float r, g, b;
-                  if(row >= height) {
-                     p -= width*comp*(row+1 - height);
-                  }
-                  if(col >= width) {
-                     p -= comp*(col+1 - width);
+                  int p;
+                  if(row < height) {
+                     p = (stbi__flip_vertically_on_write ? (height-1-row) : row)*width*comp;
+                  } else {
+                     // row >= height => use last input row (=> first if flipping)
+                     p = stbi__flip_vertically_on_write ? 0 : ((height-1)*width*comp);
                   }
+                  // if col >= width => use pixel from last input column
+                  p += ((col < width) ? col : (width-1))*comp;
 
                   r = imageData[p+0];
                   g = imageData[p+ofsG];