[Vulkan] GenerateMipmaps only generates mipmaps for the first face of a cubemap

[Doom2fan]

GenerateMipmaps seems to be only generating mipmaps for the first face (X+) in Vulkan. All other faces are left empty.

OpenGL generates mipmaps for all faces correctly, and I can't check this with D3D11 on my engine, due to this issue and this other issue in Veldrid.SPIRV. I'll try to make a simple, isolated test sample for this issue ASAP.

[Doom2fan]

Here's a hacked-up test made on top of NeoDemo. The skybox will have all zeroes for the mipmaps of faces other than X+, so it'll look like this:

D3D11 exhibits a similar issue. (#348)

[Doom2fan]

This is caused by GenerateMipmaps forgetting to account for cubemaps' ArrayLayers counting whole cubemaps and not faces. I've made a PR (#349) that fixes this and issue #346.

[Doom2fan]

This can be closed now that #349 was merged.