Speed up coverage-column-triangles test

This commit changes the height of the coverage-column-triangles test by
using a smaller height for the test. Previously the image was 40 pixels
in height and the test took 517 seconds for me. Now, the image has a
height of 4 pixels and the test takes 55 seconds.

This is possible since column_triangles() does the same thing for each y
coordinate. Thus, previously this test just did the same thing 40 times.

Times are measured with the backends I happened to have enabled locally:
script, xcb, xlib, image, recording.

Thanks a lot to Christian Rohlfs for coming up with this idea in
https://gitlab.freedesktop.org/cairo/cairo/-/issues/498#note_1700197

Possibly-fixes: https://gitlab.freedesktop.org/cairo/cairo/-/issues/498
Signed-off-by: Uli Schlachter <psychon@znc.in>

test/coverage.c

@@ -491,7 +491,8 @@ CAIRO_TEST (coverage_column_triangles,
 	    "Check the fidelity of the rasterisation.",
 	    NULL, /* keywords */
 	    "target=raster", /* requirements */
-	    WIDTH, HEIGHT,
+	    /* Smaller height since this test does not vary by y-coordinate */
+	    WIDTH, 4,
 	    NULL, column_triangles)
 CAIRO_TEST (coverage_triangles,
 	    "Check the fidelity of the rasterisation.",