Fix compiler warning (signed / unsigned mismatch)

Signed-off-by: Stefan Weil <sw@weilnetz.de>

viewer/scrollview.cpp

@@ -794,7 +794,7 @@ void ScrollView::Image(struct Pix* image, int x_pos, int y_pos) {
   int remainder = 0;
   int bits_left = 0;
   int code_len = 0;
-  for (int i = 0; i < size; ++i) {
+  for (size_t i = 0; i < size; ++i) {
     int code = (data[i] >> (bits_left + 2)) | remainder;
     base64[code_len++] = kBase64Table[code & 63];
     bits_left += 2;