testsuite: Add subpixel positioning test for hidpi

On hidpi (ie scaled by 2x) we want to align to the pixel grid, not the
unit grid.

The GL renderer currently gets that wrong, so it's disabled here.

testsuite/gsk/meson.build

@@ -142,6 +142,7 @@ compare_render_tests = [
   'stroke-opacity',
   'stroke-with-3d-contents-nogl-nocairo',
   'subpixel-positioning',
+  'subpixel-positioning-hidpi-nogl',
   'text-color-mix',
   'text-glyph-lsb',
   'text-mixed-color-nocairo',