tests: Don't check pixels outside actor allocation

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.
2 jobs for <span class="ref-name">wip/carlosg/unassorted-perf-fixes</span> in 5 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #171090
build-mutter

00:02:06

 
  Test
passed #171091
test-mutter

00:03:07