Merge branch 'alvin/T6976-angle-3.3' into krita/3.3

This adds canvas rendering with Direct3D11 via ANGLE on Windows.

ANGLE is a library which translates OpenGL ES rendering functions to
other hardware rendering APIs, including Direct3D. It is supported by
the Windows build of Qt.

This aims to improve the compatibility of the OpenGL canvas with buggy
Windows Intel drivers by allowing the use of the Direct3D11 rendering
backend of ANGLE.

Krita can still be used with a Qt build without dynamic OpenGL support,
but it will only be able to use native desktop OpenGL. To enable ANGLE,
`ext_qt` will need to be rebuilt. `ext_qt` target on Windows is also
upgraded to Qt 5.9.1, and to build it with ANGLE properly one would need
to use mingw-w64 7.1 and have Windows 10 SDK installed.

CCMAIL: kimageshop@kde.org
CCBUG: 360601
Maniphest Tasks: T5883, T6696, T6728, T6976