Check for contiguous buffer state.

When we are reading large image data (i.e.: people using webp to stream
video instead of the native webm format; I'm looking at you, Giphy!)
we can run into the situation where the available data is not in a
contiguous buffer, and we need to either buffer additional data or
re-buffer from the start. If we don't do this, we can run into issues
because of buffer over-reading (causing corrupted data if allocated or
more likely crashes if not allocated).
Re-buffering is expensive, but this should be rare and limited to
dealing with unintended use for animated image formats.

This resolves %940.

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