Fix regression with non-Vulkan backends.
Fix regression with non-Vulkan backends.

This fixes the "shader is not ASCII" error seen with GL backend,
introduced when we changed blob ownership semantics to accommodate
shader compression. It was due to missing null terminators.

We actually never bothered including the trailing null in blob length,
which was wrong but happened to work because the BlobDictionary held a
weak reference to chunk data. SInce it now holds an actual copy, the
lack of null caused our strings to contain garbage memory.

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