Mir 0.15.0

- ABI summary: Only servers and graphics drivers need rebuilding;
  . Mirclient ABI unchanged at 9
  . Mirserver ABI bumped to 33
  . Mircommon ABI unchanged at 5
  . Mirplatform ABI bumped to 9
- Enhancements:
  . Add support for Mir-on-X11.
  . Latency reduction optimizations (around ~15ms reduction in total):
    Reduced input event resampling latency by 5ms. Reduced output latency
    (in system compositors) by around 10ms with the introduction of
    "predictive bypass". And we're not finished; future Mir releases
    should reduce latency further.
  . Introduced a python3-based Mir performance framework.
  . Lots of preparation for an architectural overhaul of buffer swapping,
    required in the least to support future optimizations like nested
    bypass.
  . Added a new cursor: crosshair
  . Added support for 15/16-bit client pixel formats ("high colour").
  . Added a new client function to make picking the right pixel format
    for a given EGLConfig super simple: mir_connection_get_egl_pixel_format
  . Added application-not-responding detection
  . Added client API for specifying input region shape.
  . Fixed the remaining threading flaws identified by ThreadSanitizer and
    turned it on permanently for all continuous integration in future.
  . Added support for relative pointer motion events (e.g. for gaming).
- Bug fixes: See below.