The memory leak remains fixed, but the associated refactoring is
being reverted. Moving the initialization of the riffled palette
from png_do_read_transformations to png_init_palette_transformations
has caused a regression in some of the test programs.

Although png_init_palette_transformations is the proper place to
perform this initialization, and the test programs are technically
incorrect to fail, we are still undoing that refactoring for the
time being.