Fix crash when quantizing

Incoming pixels are negative numbers. Performing this operation
ensures we receive an accurate alpha value, without it, all pixels have negative alphas.

Fixes: 182523380
Test: Add logs, see that alphas _before_ this patch are logged
as negative numbers, causing all pixels to be marked as transparent and ignored in quantization. Add patch, see logs
indicate valid alphas.

Change-Id: Ice7b7db245de9dd089cffc490b2e24780f9c025b