Implementing filling for SkBmpCodec

Implementing filling for SkBmpCodec

The bmp codec currently returns kIncompleteInput
when the stream is truncated, which we treat as a
partial success.  However, we neglect the fill the
remaining pixels in the image, leaving these
uninitialized.

This CL addresses this problem by initializing the
remaining pixels in the image to default values.

BUG=skia:3257

Review URL: https://codereview.chromium.org/1075243003