Applies alpha channel from one texture to another texture. This applies alpha channel from maskName texture to self texture. Anything except alpha channel is ignored in maskName texture.

Caveat: As of WoW 7.0.3, this function seems to only work with builtin textures, ignoring custom textures both in self and maskName. Any attempt to pass custom texture will result in empty masked texture.

One demonstration of this is replacing builtin texture with another texture:

1. Extract Interface\Icons\Ability_AnimusDraw.blp file from WoW resources
2. Put it to <wow directory>\Interface\Icons\Ability_Ambush.blp
3. Anywhere in game where you previously saw original texture you will see replacement texture
4. Now, if you SetTexture("Interface\\Icons\\Ability_Ambush") and SetMask("Interface\\ChatFrame\\UI-ChatIcon-HotS") you will see original texture masked.