1. Changed buffer_alloc_sz and frame_size type to size_t.
2. Added a TODO for video resolution limits. On 32 bit systems, the maximum
resolution supported in the encoder is 4k(3840x2160). The malloc() would
fail if encoding >4k video on a 32 bit system.

Change-Id: Ibd91b28fd63d1b04e8ac9a5270a17629f239188a