Avoid trying to create empty DrawTarget for painting range input shadow.

After bug 1918838 we can get a way smaller DPIRatio which ends up with a
zero-sized draw target. I don't think we care much about not drawing the
shadow in that case.

Differential Revision: https://phabricator.services.mozilla.com/D228074

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1928736
gecko-commit: dd1e7d287b6651b30511ff6991e87871b7101c6a
gecko-reviewers: gfx-reviewers, bradwerth

css/css-masking/mask-image/firefox-bug-1928736-crash.html

@@ -0,0 +1,10 @@
+<style>
+input:in-range {
+  mask-image: url(#a);
+}
+* {
+  zoom: 32%;
+}
+</style>
+<canvas id="a"></canvas>
+<input type="range">