function queryPreviewProviders(editor, pos, token) {
        var line = editor.document.getLine(pos.line);

        // FUTURE: Support plugin providers. For now we just hard-code...
        var popover = colorAndGradientPreviewProvider(editor, pos, token, line) ||
                      imagePreviewProvider(editor, pos, token, line);

        if (popover) {
            // Providers return just { start, end, content, ?onShow, xpos, ytop, ybot }
            popover.visible = false;
            popover.editor  = editor;
            return popover;
        }

        return null;
    }
Returns a 'ready for use' popover state object:
{ visible: false, editor, start, end, content, ?onShow, xpos, ytop, ybot }
Lacks only hoverTimer (supplied by handleMouseMove()) and marker (supplied by showPreview()).