[Vulkan] GenerateMipmaps generates strange mipmaps

[Doom2fan]

GraphicsDevice.GenerateMipmaps doesn't seem to be working too well.
It looks as if it's trying to use the first mip level to generate all the mip levels, which is incorrect and leads to heavy aliasing in the mipmaps themselves.

OpenGL's mipmaps seem correct, 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 for this issue. It's hacked onto NeoDemo. This is what it should look like:

[Doom2fan]

This is, indeed, caused by incorrectly using the first mip level as the source of all the other mip levels. I've made a PR (#349) that fixes this and issue #347.

[Doom2fan]

This can be closed now that #349 was merged.