Shader cache not able to write out compiled shaders

Currently the Ubuntu Touch OS is very restrictive about what paths can be accesed by each application.

src/gui/opengl/qopenglprogrambinarycache.cpp tries to write to the general cache folder on the users home dir level, but that is blocked: https://github.com/qt/qtbase/blob/823ed71e220ebde07970dd61c04bb47b01dd06c4/src/gui/opengl/qopenglprogrambinarycache.cpp#L416-L423 fails. QStandardPaths::GenericCacheLocation is write-protected unless an exemption is made for an app or for all apps.
We are instead offering the applications to write to QStandardPaths::CacheLocation but this is not used by the shader cache code. And we believe this is a bug actually.