ANGLE Texture Sharing

Available only on the OpenGL ES backend when running on top of ANGLE. When a device is created with this extension, it is added to the EGL_DISPLAY_TEXTURE_SHARE_GROUP_ANGLE which is a display-wide share context, but only for texturs.

When enabled, dawn::native::opengl::ExternalImageDescriptorGLTexture and dawn::native::opengl::WrapExternalGLTexture become available which can import a GL texture into a wgpu::Device with a GL texture ID.

See the spec for EGL_ANGLE_display_texture_share_group.