fix rescaling bug: alpha plane wasn't filled with 0xff

(in case no alpha was present in the source .webp,
but user requested some)

Change-Id: I9011d38237907c60d6796a86bd2c72166aa80f27
(cherry picked from commit 06a38c7b1cffeeb8216efb465fafc1c40d1150d7)
1 file changed