Anki add-on: Text Rendering Fix

NOTE: This is fixed in newer versions of Anki 2.1. This addon is only needed for people who use old versions of Anki 2.1. Fixes the lack of subpixel anti-aliasing on Anki 2.1 by overriding the color set as QWebEnginePage's background. Vanilla Anki 2.1 uses Qt.transparent, which prevents subpixel anti-aliasing from working. Exact background color is configurable. Before: After: Should apply to all platforms that support subpixel text filtering.