Fix panic on wgpu GL backend due to new screenshot capability

[amfaber]

This is a first-pass triage for #3068. The fix involves simply disabling the screen shot feature when using the GL backend with wgpu. If a screenshot is requested on this backend, None will be provided and an error is logged out. The docs have been updated with an small comment on how the feature isn't available with the GL backend.

The reason that the feature is incompatible with the GL backend is that this backend doesn't allow the surface to have the TextureUsages::COPY_DST flag, and so the approach of rendering to a texture and subsequently copying the result to the surface and a buffer for extracting to the CPU is fundamentally incompatible.

Perhaps @Wumpf can help with ideas on how to circumvent this?

[Wumpf]

Great catch, makes sense!

But the culprit isn't really GL backend not supporting a thing, but that that we blindly assume the COPY_DST usage is supported. This can happen on any backend in theory, so therefore we should check the availability of that usage flag and then draw the right conclusion.... only that I noticed now that this is not possible right now, I opened an issue here gfx-rs/wgpu#3869

Even better would be ofc to just not use texture copies here, but instead do screen triangle pass to render the final texture, but let's not get into additional complexities right now :)

So given the situation I think we can move on with this, but I'd like some small improvements in here, see comments :)

[Wumpf]

ci is unhappy, otherwise good to go imho

[amfaber]

Thanks for the input! As I am still getting into graphics programming, I am especially interested in the benefits of doing a triangle screen (re) render rather than a copy: Are there any benefits other than not requiring COPY_DST on the surface (though this benefit is clearly sufficient on its own)? In my mind I would have thought that a straight copy would be one of the most efficient operations that we could perform. Really looking to learn ^^

[Wumpf]

It should generally be way better to do the copy via the copy texture operation, but with these things it's very much dependent on the driver and what other loads are in flight. But yes the only actual reason here would be compatibility