Fix two new Visual Studio compiler warnings

src/Fl_SVG_Image.cxx

@@ -119,7 +119,7 @@ static char *svg_inflate(gzFile gzf, // can be a file or the read end of a pipe
   do {
     if (is_compressed && p + size > out + out_size) {
       out_size += size;
-      unsigned delta = (p - out);
+      size_t delta = (p - out);
       out = (char*)realloc(out, out_size + 1);
       p = out + delta;
     }

src/fl_draw.cxx

@@ -79,8 +79,8 @@ static const char* expand_text_(const char* from, char*& buf, int maxbuf, double
     if (o > e) {
       if (maxbuf) break; // don't overflow buffer
       l_local_buff += int(o - e) + 200; // enlarge buffer
-      unsigned delta_o = (o - local_buf);
-      unsigned delta_end = (word_end - local_buf);
+      size_t delta_o = (o - local_buf);
+      size_t delta_end = (word_end - local_buf);
       local_buf = (char*)realloc(local_buf, l_local_buff);
       e = local_buf + l_local_buff - 4; // update pointers to buffer content
       o = local_buf + delta_o;