plugins/qpa: set deprecated functions option correctly
If a context is forward compatible, that means the deprecated functions are not available, and if the QSurfaceFormat::DeprecatedFunctions option is set, that means they are available. Wrongly setting QSurfaceFormat::DeprecatedFunctions thus causes Qt to use OpenGL in a way the context doesn't actually support.
(cherry picked from commit 28059435)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com