The actor-shader-effect test actors are 50px wide, but we check the 51st
pixel. This went along undetected until "clutter: Avoid rounding
compensation when invalidating 2D actors" because the paint volumes were
made slightly bigger and the shaders paint all over them (I guess nobody
noticed those actors being actually ~52px wide).
Update the test to check the middle of the opposite edge, so we keep neatly
rounded numbers.
(cherry picked from commit 1d73533f78)
