Implement missing depth/stencil formats.

Implement missing depth/stencil formats.

Several depth-only formats were implemented using depth+stencil
formats. This resulted in unintended stencil operations happening, as
well as unnecessary allocations and blitting of stencil data.

This change also introduces quad-layout depth+stencil formats for more
efficient rendering.

Note that as before the 'internal' buffer's format also indicates the
presence of a stencil component, even though it only stores the depth
component, while the 'stencil' buffer now has an S8 format or NULL for
a depth-only format.

Change-Id: I245f0cb5a999851e24082f3ab1ea78a5f5956af3
Reviewed-on: https://swiftshader-review.googlesource.com/14988
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>