The stencil buffer always has a quad layout, and can't be copied using
copyBuffer() even when copying the full buffer, because it copies one
row at a time (i.e. it assumes a linear layout).

Likewise a depth buffer may optionally have a quad layout and copying
the full buffer still doesn't allow using copyBuffer().

To keep maximum performance for the typical case of copying between
equally sized buffers, a new code path it added to copy the entire
slice in one memcpy() call.

Change-Id: I77f60f66e5cdcd7cf0a73d03f3d3485809d7ebe6
Reviewed-on: https://swiftshader-review.googlesource.com/15008Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>