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Since we can't compute the color difference of an outline point if it's
outside the background bounds, we will simply ignore it in the weighted
average. This removes the artifacts that appeared from such cases.

Note that this doesn't solve the problem where all selected sample
points are outside the background, but the algorithm isn't really meant
for handling such cases so this is the best we can do.

In the seamless-clone-prepare op, warn if the destination pointer for
the result is NULL. Also, actually store the pointer to the result at
the given pointer to a pointer.

In the seamless-clone-render op, rename the offset properties to "x"
and "y" instead of "xoff" and "yoff".

This commit also includes the code of the poly2tri-c library.

docs/index.txt would be deleted by a `make clean`. It must not be versionned.
Several other generated .txt files were dirtying a git status.

transform-core.c: move handling of matrix coefficients to gdouble and fix is_zero, is_one and matrix3_is_affine

transform-core.c: restore {0,0,0,0} and adding the whole width and height in gegl_transform_get_bounding_box