Changes Emscripten SDK version to match that used to compile the default export
templates used by Godot 3.5 (in this case 3.1.14 as can be seen here:
https://github.com/godotengine/build-containers/blob/3.5/Dockerfile.javascript).
This means we no longer need to compile custom export templates when
exporting HTML5 for Godot 3.5. Exports from other Godot versions may
requiring compiling the GDNative library with a different version of the
Emscripten SDK.
- Changes renderer from GLES3 -> GLES2 and reduces MSAA level to better
support HTML5 export.
- Updates GitHub Action to export HTML5 and upload as a build artifact.
- Adds Cypress test to smoke test HTML5 export.
Adds support for building Linux binaries inside a docker container in
order to target an older version of GLIBC.
Updates GitHub Actions workflow to use it.
As a result the minimum version of GLIBC that Linux users need to have
installed on their system is 2.17 which was released in 2012.
