Warn about missing/incorrect --scale-factor CSS-variable in renderTextLayer (issue 16139)

Unfortunately I don't believe that we can simply add a default `--scale-factor` CSS-variable to the `container`-element, since that might not be entirely appropriate/correct in all cases.[1]
However, we can at least print a console-error to hopefully make this situation more apparent to users. (This is purposely not using the `warn` helper-function, since those messages can be disabled.)

---
[1] One example is in our reference-tests, where we don't need to add it to the `container`-element itself.

test/unit/text_layer_spec.js

@@ -35,7 +35,7 @@ describe("textLayer", function () {
     const textLayerRenderTask = renderTextLayer({
       textContentSource: page.streamTextContent(),
       container: document.createElement("div"),
-      viewport: page.getViewport(),
+      viewport: page.getViewport({ scale: 1 }),
       textContentItemsStr,
     });
     expect(textLayerRenderTask instanceof TextLayerRenderTask).toEqual(true);