Revert "Set default resolution to 1280x1024 now that reftests render to FBO."

This reverts commit c74798bb6bcb98d2d6af998e2ab0f08e79097147.
auto merge of #3517 : glennw/servo/revert-res-change, r=metajack

...FBO."

This reverts commit c74798bb6bcb98d2d6af998e2ab0f08e79097147.