This is a bit of a switch-up, see this comment for more background:
refs: https://github.com/wez/wezterm/issues/265#issuecomment-701882933
This commit:
* Adds a pre-compiled mesa3d opengl32.dll replacement
* The mesa dll is deployed to `<appdir>/mesa/opengl32.dll` which by
default is ignored.
* When the frontend is set to `Software` then the `mesa` directory
is added to the dll search path, causing the llvmpipe renderer
to be enabled.
* The old software renderer implementation is available using the
`OldSoftware` frontend name
I'm not a huge fan of the subdirectory for the opengl32.dll, but
I couldn't get it to work under a different dll name; the code
thought that everything was initialized, but the window just rendered
a white rectangle.
