Lost an ability to use external GLES context on Windows

[kvark]

Description
I was using wgpu-0.17 in vange-rs, and specifically the method new_external with Gles backend. It allowed me to create the context with SDL and pass to wgpu. In later versions, however, the whole module got disabled on Windows. I'm not seeing an alternative right now.

Repro steps
kvark/vange-rs#246

Expected vs observed behavior
Expected to keep this feature.

Platform
Windows, currently trying wgpu-0.19

[teoxoy]

This is probably due to #4248 but #6152 added new_external for WGL as well. Could you give that a try?