Various fixes to new Cards

Summary:
This contains various small fixes to the new Cards.

• Still show the border of AbstractCard on Cards with images.
• Round bottom corners of the banner when the card doesn't have any other contents.
• Don't use fwidth for smoothing, instead just use a fixed value, this increases the accuracy of lines rendered by cards.
• Fix batching test for ShadowedTexture, prevents cards being rendered with the wrong texture.

BUG: 420406

Test Plan: The new Kirigami Gallery displays more correct now, though there are some layout issues that are not regressions.

Reviewers: Kirigami, ndavis, mart

Reviewed By: Kirigami, ndavis, mart

Subscribers: ndavis, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D29092