Remove image size rounding to 4x4 multiple.

Bug 21935792

Surface::size() already takes into account that compressed images
need their dimensions rounding up to whole 4x4 tiles.

Change-Id: Ie6b7b5d23294e8d3c85b3b362670cfe0c569585d
Reviewed-on: https://swiftshader-review.googlesource.com/3571
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>