Regression: Burmese rendering issues under Qt 5.6

Under Qt 5.6, Burmese rendering of text based on the ZawgyiOne font is broken. It used to render properly under Qt 5.5.

The Qt libraries offered by Ubuntu are compiled using the -system-harfbuzz flag. I've tested the system harfbuzz library (1.2.7.1) using hb-view, and confirmed that the rendering of Burmese text using ZawgyiOne font is working properly there. This effectively rules out a harfbuzz library regression.