It is actually possible to do using a feature called dual source blending, which unfortunately is so rarely used that drivers tend to fail to implement it prope

It is actually possible to do using a feature called dual source blending, which unfortunately is so rarely used that drivers tend to fail to implement it properly. But it does give you the possibility to use 3+ alpha channels with full hardware acceleration, at some memory cost. Though since DPIs are going up and 3 alpha channels is such an unusual thing, I guess they just figured it wasn't worth the trouble.