overrideInternal with texture arrays + layer index

[bghgary]

We have a need to be able to use an externally provided D3D texture to bgfx where the D3D texture is a texture array. One example where the source texture is an array is when it is coming from a media foundation video. In order to select the correct texture from the array, we need to set the FirstArraySlice of the D3D11_TEX2D_ARRAY_SRV. Since the D3D shader resource view is created inside bgfx's overrideInternal function, we need a way to set this property there.

I mocked up an implementation that does this, but it's clunky because it's D3D specific and overrideInternal is obviously not. I only implemented it for D3D11 for now.

@bkaradzic Any thoughts on whether this is okay or not?

[bkaradzic]

Yeah it looks good. I just don't think you need to use _layerIndex -1 to signify anything.