Add RtL support for draw_text_attachment_to_canvas

[momo-ozawa]

Description

Text alignment for screenshot overlay text must be flipped for RtL languages.

Background Context

Ref: wordpress-mobile/WordPress-iOS#16707

While adding the ar-SA (Arabic) and he (Hebrew) locales to screenshot generation, I realized that the overlay text alignment needs to to be flipped for RtL languages.

In the Jetpack app, the splash image has text that's left-aligned for LtR languages, which needs to be flipped to be right-aligned for RtL languages.

JP