add CI workflow for MSVC and fix GDI demo

[sleeptightAnsiC]

Notes:

• setting up Windows/MSVC manually would be probably a huge headache. I just reused ilammy/msvc-dev-cmd@v1 action which seems to be the most popular one.
• every demo/*/build.bat hardcodes directory to the same VC2015 toolchain. That's why CI uses arch: x86 and toolset: 14.0. Would be nice to clean it up someday but for now I just want to get it in
• the workflow is in separate file. I thought it makes sense since it needs to run under entirely different OS/toolchain and builds entirely different demos
• GDI demo appears to be broken since 0639685. I've patched it in separate commit.
• build d3d11 step is noticeably slower. I don't have any clear explanation for this. Maybe because it is first to run, maybe because of different shader compilation, I'm not sure...
• there are quite a lot of warnings. I may fix those later as part of Compiler warnings #958
• I'm a total noob when it comes to Github CI, so maybe there is something obvious I didn't notice
• DO NOT SQUASH

Refs: #895
Refs: #937