Fix: ws_buffer_blit() should take const source
Implement blit on commit
Change: Multi test setup for tests/check/objects

This patch changes the testing setup from one-file-one-executeable for
the object tests to a multi-file test setup.

The idea is the very same as for the top-level cmake file: Include all
subdirs specified by a list.

The toplevel cmake file is adapted.
Simplify list by moving prefix in iteration
Merge pull request #183 from matthiasbeyer/change-to_multi_file_tests_for_objects

Change test build setup to allow one test per object type
Merge pull request #173 from matthiasbeyer/add-commit_blitting

Implement blit on commit