Use only external surface dimensions.

They should always be identical to the internal ones.

Bug 21424351

Change-Id: Iafa18c14cc4d2e16f20238dac0208a8159cbaa6a
Reviewed-on: https://swiftshader-review.googlesource.com/3460
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>