structure in the libpng documentation.

handling. Internal changes only except for a new option to avoid compiling
in the png_build_grayscale_palette API (which is not used at all internally.)
The main change is to move the transform tests (READ_TRANSFORMS,
WRITE_TRANSFORMS) up one level to the caller of the APIs.

that was introduced in libpng-1.5.2beta01.

(compression_level -> compression_method)

(see libpng-1.2.6beta03).

compression

except for strategy which defaults to the zlib default.

is supported.