Image mapping now returns the map pointer of a separate
buffer, rather than the the image itself. This fixes
issues with applications that ignore the RowPitch
and/or DepthPitch fields of the MappedSubresource struct.
This is required because in D3D11, typeless formats can be used
to create both depth and stencil images, and color formats can
be used to view depth images. In Vulkan, images and views that
are used as depth-stencil attachments will have to be created
with a depth-stencil format, so we have to take the image's
bind flags into account when picking a format.
