Add lossless_big_random_alpha.webp

compressed with v0.5.1-70-g386e4ba:
$ ./cwebp -lossless -exact

Change-Id: Ifd14209660e0314e139ff34c8b638c1945ddabe3
diff --git a/lossless_big_random_alpha.webp b/lossless_big_random_alpha.webp
new file mode 100644
index 0000000..7591e71
--- /dev/null
+++ b/lossless_big_random_alpha.webp
Binary files differ