fix bug in RefineUsingDistortion()
fix bug in RefineUsingDistortion()

When try_both_modes=0 (that is: -m 0 or -m 1), and the mode is i4,
we were still sometimes falling back to (unexplored, uninitialized) i16 mode,
which resulted in a enc/dec mismatch.
This was mainly occurring for large images (when bit_limit is low enough)

We disable the fall-back by disabling bit_limit using a large MAX_COST threshold.

Change-Id: I0c60257595812bd813b239ff4c86703ddf63cbf8

webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp