Add subsurface.

A subsurface is a region of another surface that may be used either to
restrict the writable area of a context or the readable extents of a
source. Whilst writing, access to the exterior of the subsurface is
prevented via clipping and when used as a source reads from the exterior
of the subsurface are governed via the extend mechanism of the pattern.

1 files changed, 0 insertions, 0 deletions

diff --git a/test/subsurface-repeat.ref.png b/test/subsurface-repeat.ref.png
new file mode 100644
index 000000000..c37e22e36
--- /dev/null
+++ b/test/subsurface-repeat.ref.png