Allow to skip highest-resolution encoding in multi-resolution encoder

Sometimes, a user doesn't have enough bandwidth to send high-resolution
(i.e. HD) video even though the camera catches HD video. This change
allowed users to skip highest-resolution encoding by setting that level's
target bit rate to 0.

To test it, modify the following line in vp8_multi_resolution_encoder.c.
    unsigned int  target_bitrate[NUM_ENCODERS]={1400, 500, 100};
To skip the highest-resolution level, change it to
    unsigned int  target_bitrate[NUM_ENCODERS]={0, 500, 100};
To skip the first and second highest resolution levels, change it to
    unsigned int  target_bitrate[NUM_ENCODERS]={0, 0, 100};

This change also fixed a small problem in mapping, which slightly helped
quality and performance.

Change-Id: I977bae9a9fbfba85c8be4bd5af01539f2b84bc81