Enable Alpha8 render targets

[kg]

I vaguely recall this working in XNA - I think escape goat 2 used it. Right now it works for D3D11 and Vulkan with this change, but the behavior is different when sampling from the resulting RT.

D3D11:

Vulkan:

Maybe an sRGB vs linear thing? Not sure.

[kg]

The difference is in the way blending/rendering works for the RT; sampling is consistent. Disabling blending when drawing to it makes it work the same on both.

[flibitijibibo]

Let's add this when gpu is ready, we should make sure it works there first.