switched from identity to file

ImageMagick is no longer required to run the tests

Author: moonglum

test/run

@@ -46,10 +46,10 @@ function assert_svg {
 	grep -q svg "${1:?}" || fail "expected $1 to be an SVG"
 }
 
-# checks the dimensions of an image, requires ImageMagick
+# checks the dimensions of an image
 function assert_dimensions {
 	expected_dimensions="${1:?}"
-	actual_dimensions=$(identify -format "%wx%h" "${2:?}")
+	actual_dimensions=$(file "${2:?}" | grep -oE "[0-9]+x[0-9]+")
 
 	if [ "$expected_dimensions" == "$actual_dimensions" ]; then
 		true