Pure GL context

[kvark]

Is your feature request related to a problem? Please describe.
Sometimes we want to integrate wgpu into a system that already created the window and GL context, and we want to draw something using wgpu's GLES context in addition to other stuff. In this case, EGL isn't involved.

Describe the solution you'd like
Some sort of constructor for GLES adapter, used in conjunction with create_adapter_from_raw.
This also means working without any Instance, and without any EGL involved.
Would be great to make it not a compile time feature.

Describe alternatives you've considered
Separate cargo feature for "raw" mode.

Additional context
required for kvark/vange-rs#172

[caiiiycuk]

This also can be used with emscripten target.