This fixes two issues. We had a global 'blitter' instance which doesn't
get destroyed until the library us unloaded. This is reported as a
memory leak (albeit benign). Potentially worse is that the singleton was
shared between all threads, and the Routine cache isn't thread safe.

Bug chromium:732691

Change-Id: I7b90d7d2bc67b4a9f78cdf4f54a76fa2f798b7cc
Reviewed-on: https://swiftshader-review.googlesource.com/10128Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>