This MR adds a custom ANGLE build ext_googleangle to 3rdparty and makes Qt build against it instead of it's bundled ANGLE. The HDR support patch by @dkazakov has also been rebased.
The custom ANGLE build uses a CMakeLists.txt taken from vcpkg and adapted to this version of ANGLE.
The ANGLE version used is the one that goes with Chromium release 100 (currently in Beta channel). See 3rdparty/ext_googleangle/CMakeLists.txt for more details on the selected ANGLE revision.
The current ANGLE patches correspond to this branch: https://github.com/alvinhochun/angle/tree/for-krita
This MR depends on !1372 (merged), because building this ANGLE needs (1) updated mingw-w64 headers and (2) Clang. In theory the mingw-builds GCC 11.2 toolchain can work, but I found that GCC generates broken code with latest ANGLE. (It can be worked around in code but I haven't looked into that yet.)