bits: optimise fetching width==1 repeats

bits: optimise fetching width==1 repeats

Profiling ign.com, 20% of the entire render time was absorbed in this
single operation:

<< /content //COLOR_ALPHA /width 480 /height 800 >> surface context
<< /width 1 /height 677 /format //ARGB32 /source <|!!!@jGb!m5gD']#$jFHGWtZcK&2i)Up=!TuR9`G<8;ZQp[FQk;emL9ibhbEL&NTh-j63LhHo$E=mSG,0p71`cRJHcget4%<S\X+~> >> image pattern
  //EXTEND_REPEAT set-extend
  set-source
n 0 0 480 677 rectangle
fill+
pop

which is a simple composition of a single pixel wide image. Sadly this
is a workaround for lack of independent repeat-x/y handling in cairo and
pixman. Worse still is that the worst-case behaviour of the general repeat
path is for width 1 images...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>