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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