platform: add deprecation path for mg::BufferUsage for the eglstream platform. This adds a path to create a software buffer.
Also add a unit test infrastructure for the eglstream platform (was missing)
note: this is part of the effort trying to pull MirBufferUsage from client api for 1.0.
Approved by mir-ci-bot, Alan Griffiths, Cemil Azizoglu.