[BITSTREAM]Fix the scaling calculation

For very large size video image, the scaling calculation may need use
value beyond the range of int. This commit upgrade the value to 64bit
to make sure the calculation do not wrap around INT_MAX.

The change corrected the decoder behavior.

The bug affects only very large resolution video because the scaling
calculation was sufficient for image size smaller than 2^13.

This resolves issue:
https://code.google.com/p/webm/issues/detail?id=750

Change-Id: I2d2ed303ca6482f31f819f3c07d6d3e98ef3adc5