icat: Allow controlling how images are fit

Fixes #9201

kittens/icat/scaling_test.go

@@ -0,0 +1,29 @@
+package icat
+
+import (
+	"fmt"
+	"image"
+	"testing"
+)
+
+var _ = fmt.Print
+
+func TestScaling(t *testing.T) {
+	for _, tc := range []struct {
+		w, h, pw, ph, ew, eh int
+	}{
+		{1000, 50, 800, 600, 800, 40},
+		{1000, 50, 800000, 600, 12000, 600},
+		{100, 50, 800, 600, 800, 400},
+		{1920, 1080, 800, 600, 800, 450},
+		{300, 900, 800, 600, 200, 600},
+		{400, 300, 800, 600, 800, 600},
+	} {
+		aw, ah := scale_up(tc.w, tc.h, tc.pw, tc.ph)
+		actual := image.Pt(aw, ah)
+		expected := image.Pt(tc.ew, tc.eh)
+		if actual != expected {
+			t.Fatalf("want: %v got: %v", expected, actual)
+		}
+	}
+}