recorder: Always initialize prev when computing the run-length
recorder: Always initialize prev when computing the run-length

We need to initialize prev when we handle the initial pixel in a
rectangle, or we may detect the following pixel as identical or different
when it's not.  This causes the top-left pixel in a rectangle to
occasionally be wrong leaving a trail of "dirty pixels" in the capture.

mchalupa/weston,sir-murray/weston,udoprog/weston,giucam/weston,krezovic/weston,xorgy/weston,udoprog/weston,sir-murray/weston,xorgy/weston,jonnylamb/weston,kwm81/weston,Fantu/compositor-spice,mchalupa/weston,Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,Gnurou/weston,giucam/weston,Fantu/compositor-spice,eyolfson/weston,jonnylamb/weston,kwm81/weston,krezovic/weston,eyolfson/weston,Gnurou/weston