Bug 1023473. Pad out tile contents by one pixel. r=BenWa

This fixes reading unitialized data during texture filtering.
It's complicated because we can have regions of initialized data
and padding out a region is much harder than a rectangle.

We currently only take this path for tiled things because of
the artifacts that show up during the sampling that helps
in overscroll. It's also not a great long term solution because
it only works for software backends.