ETC2 support for GL backend

[tasogare3710]

Description

ETC2 is required if GL4.3 and higher or GL_ARB_ES3_compatibility extension is available.

Repro steps

Specify Features::TEXTURE_COMPRESSION_ETC2 for DeviceDescriptor::required_features.

let (device, queue) = adapter
    .request_device(
        &wgpu::DeviceDescriptor {
        required_features: wgpu::Features::TEXTURE_ADAPTER_SPECIFIC_FORMAT_FEATURES
                         | wgpu::Features::TEXTURE_COMPRESSION_ETC2,
        ...
        },
        None,
    )

Expected vs observed behavior

If ETC2 is supported by the GL backend, I want to use ETC2, but currently I get the following error when I try to enable ETC2.

Failed to create device: RequestDeviceError { inner: Core(UnsupportedFeature(Features(TEXTURE_COMPRESSION_ETC2))) }

Extra materials

wgpu/wgpu-hal/src/gles/adapter.rs

Lines 507 to 508 in 990324f

	// This is a required part of GLES3, but not part of Desktop GL at all.
	es_ver.is_some()

L508 can be fixed as supported((3, 0), (4, 3)) || extensions.contains("GL_ARB_ES3_compatibility");.

I think there is a wrong in not part of Desktop GL at all. Or is there some reason?

Platform

• Windows [Version 10.0.19045.3930]
• Intel HD Graphics (Haswell GT1)
• GL_VERSION: "4.3.0 - Build 20.19.15.5171"
• GL_SHADING_LANGUAGE_VERSION: "4.30 - Build 20.19.15.5171"
• GL_RENDERER: "Intel(R) HD Graphics"
• GL_VENDOR: "Intel"