Add disable_srgb to NativeOptions.

[ar37-rs]

Hi @emilk eframe broken again on virtualbox gl es 2.0 since #2070 (probably because it has GL_EXT_texture_sRGB_decode, but doesn't do anything).
adding disable_srgb will avoid srgb textures support misdetection.

[emilk]

I want to merge #2071 before merging this, but a few quick comments:

• Don't use negations in names. Call it enable_srgb_textures: bool instead
• I don't see a need for Option<bool>. Just use bool.

Is it possible to detect virtualbox some other way? i.e. via gl.get_parameter_string(glow::SHADING_LANGUAGE_VERSION) or similar?

[ar37-rs]

@emilk exactly, agree on that. since an OpenGL ES 2.0 driver usually has no SRGB8_ALPHA8 support, what if we just disable srgb by default if the shader version detected is egui_glow::ShaderVersion::Es100 ? that would be much more simpler.

[ar37-rs]

Like so:

        let is_webgl_1 = shader_version == ShaderVersion::Es100;
        //...
        let srgb_textures = if is_webgl_1 { false } else { true };
        tracing::debug!("SRGB Support: {:?}", srgb_textures);

since webgl1 is equivalent to GLES2 and webgl2 is GLES 3.0

[ar37-rs]

@emilk current master woking fine on virtualbox, thank you.