Most client code no longer needs to care about paddings at all, aside
from some very special cases involving dynamic creation of padded
sprites. So far we only have one known example of this in portrait_render().
This should let us optimize most of our sprites with autotrimming in the
future.
Additionally, support for deprecated offset_{x,y} keys in sprite files
has been removed.
That would trip an assertion. May happen with the null renderer (e.g.
replay verification mode), where the fake textures are always 1x1, or
with bad sprite data.
