lib/igt_fb: Add igt_put_cairo_ctx as counter to igt_get_cairo_ctx

This will allow support for NV12 in the future, where igt_get_cairo_ctx
will return a RGB image to draw with, which will be converted in
igt_put_cairo_ctx so tests don't have to add special support for NV12.

This is the same as cairo_destroy + checking for errors, but not all
tests use this correctly so it's better to have a single handler for it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
16 files changed