This creates the FP16 shadow framebuffer automatically if the color
transformation from blending space to output space is not identity and
the backend does not claim to implement it on the renderer's behalf.
That makes the weston_output_set_renderer_shadow_buffer() API and
use-renderer-shadow weston.ini option obsolete.
To still cater for the one test that needs to enable the shadow
framebuffer in spite of not needing it for color correct blending, the
quirk it uses now also forces the shadow.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>