Changing input method with shortcut key causes hang on WebEngine

To reproduce:

• Start bar>Search for 'language settings', and ensure there are two available languages
• Start bar>Search for 'advanced keyboard settings'
• In subsequent window, click on 'language bar options', then the 'advanced key settings' tab
• Set up hot keys to switch between the two languages, such as shift+alt+1 and shift+alt+2
• Run the webengine markdown sample app on a system that supports hardware acceleration (tested on a system with an Nvidia card, and one with Intel graphics)
• Press the shortcut key for the input method that's not currently enabled, and observe the app has hung.

Notes:

• On systems which don't support hardware acceleration, or when QT_OPENGL is set to software, the issue can't be reproduced.
• A user reported an earlier build with Qt 5.9 doesn't exhibit the issue. Another user's report seems to indicate the issue exists with Qt 5.9 as well.