libliftoff v0.2.0

This is the second libliftoff release. Major changes:

- All functions which can fail now return zero on success, and a negative error
  number on failure. This allows callers to get more information about failures.
- liftoff_output_needs_composition and liftoff_layer_needs_composition have been
- libliftoff has migrated to

Full commit history below.

Simon Ser (41):
      Make liftoff_output_apply return an int
      Make liftoff_device_register_all_planes return an int
      Stop including stdbool.h from libliftoff.h
      Make liftoff_layer_set_property return negative errno
      build: bump version to 0.2.0
      Add needs_composition APIs
      Improve prop value pretty-printing in output_log_layers
      Make fp16_to_double static
      Log when no layer has been a plane
      Log number of candidate planes
      test: make sure NDEBUG is not defined
      Log number of layers in output_log_layers
      Log when a test-only commit fails
      Print alloc debug logs as a tree
      Zero-initialize alloc_step and alloc_result
      Improve alloc debug logging
      test: introduce liftoff_mock_require_primary_plane
      Document why the primary plane is a special case
      build: turn on additional warnings
      Add newline after function return type
      Remove test/include/
      Log number of atomic test-only commits
      test/test_prop: add unmatched test case
      Make plane_get_property static
      ENOSPC is not fatal for test-only commits
      Migrate to
      ci: drop py3-setuptools from Alpine packages
      ci: add .gitlab-ci.yml
      Check layers zpos over the primary plane
      test/alloc: add zpos-2x-reverse
      test/alloc: add zpos-2x-fail
      test/alloc: make test data const
      test/alloc: check test_layer.compat length
      test/alloc: replace test_layer.zpos with test_prop
      test/alloc: pass full test_case to run_test
      test/alloc: drop test_needs_composition logic
      test/alloc: move zero-fb-id-fail to test data
      test/alloc: rename composition-zero-fb-id to composition-no-props
      Ignore invisible layers in layer_intersects
      test/alloc: add zpos-3x-zero-fb-id
      test/alloc: add zpos-3x-zero-alpha