Move bounds checks for REPEAT_NONE to get_pixel()

Move bounds checks for REPEAT_NONE to get_pixel()

On a P4, this is a large speedup for the swfdec-fill-rate-2xaa trace:

After:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image        swfdec-fill-rate-2xaa   33.061   33.061   0.00%    1/1

Before:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image        swfdec-fill-rate-2xaa   40.342   40.342   0.00%    1/1

Pixman 0.14.0 produces this:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image        swfdec-fill-rate-2xaa   36.896   36.896   0.00%    1/1