Bug 916535. Avoid using repeating gradients for tiling when it won't work. r=roc

Avoiding normalization introduced in bug 895135 caused gradientStart/Stop to
not correspond to the firstStop and lastStop. This can cause us to incorrectly
decide to use the repeat fast path. This patch switches takes us back to the
same condition as we had before the regression.

--HG--
extra : rebase_source : f335e1db9166213115354eb14dbe90b09127a68b