alpha masking doesn't work when drawing to canvas

[nadako]

originally FOE-55746

Reproduction Steps:

have a bitmap with alpha pixels
use it as a mask for something
draw that something to a bitmap or use in a cacheAsBitmap'd container

Actual Behavior:

alpha pixels are not considered empty for masking (so the mask is always rectangular)

Expected Behavior:

alpha pixels must be considered (at least if both mask and masked object are cacheAsBitmap'd, see https://help.adobe.com/en_US/as3/dev/WS5b3ccc516d4fbf351e63e3d118a9b90204-7e0b.html#WS2d8d13466044a7332384449d1277856ffe8-8000