feat: sub-frames

[Drakulix]

Description

Alternative to #1935.

Old PR

This PR implements Renderer for the GlesFrame. This allows "sub"-frames to be created, which temporarily borrow the original GlesFrame to render to a different framebuffer, before restoring the previous framebuffer. Additionally this allows other traits such as Import* to be implemented for the GlesFrame easily.

There is a few things missing before this PR could be considered for merge:

• Various TODOs
  • Try to avoid some unnecessary make_current-calls. Though even an active GlesFrame is technically no guarantee, that there isn't another GlesFrame from a different context...
  • Make texture-filter and debug-flags functions/variables local to a specific frame and not for the context
• Create some GlesRendererLike-trait, so that RenderElements, such as the TextureShaderElement or the PixelShaderElement can be implemented generically across both GlesRenderers and GlesFrames.
• Allow subframes of GlowFrames just the same.
• Investigate if we can (and want to) implement this for PixmanRenderer, MultiRenderer and TestRenderer as well.
• Make sure this is actually sound (there are a few raw-pointers in there, to allow indefinitely nested GlesFrames even though we need to borrow the internal GlesTarget as well to be able to restore it.

Last but not least, there might be options to implement this in a more straight-forward manner:

• We could potentially borrow a GlesRenderer from a GlesFrame with some kind of Guard-struct that restores the target again on drop. This would require fewer impl-blocks and adjusted implementations.
• If we don't actually need sub-frames, but just Import* or Offscreen (as renderer/gles: Expose create_texture() on GlesFrame #1935 suggests), we could also change those to only depend on RendererSuper and implement that for Frame instead of Renderer-itself.

Feedback is very much wanted. (Ping @YaLTeR, @cmeissl, @ids1024)

EDIT: New Approach

A new Frame-trait (please bikeshed on the name) to allow borrowing the old renderer and then restore the current frame on drop.

Implemented for all renderers, but needs testing.

Checklist

• I agree to smithay's Developer Certificate of Origin.

[YaLTeR]

• If we don't actually need sub-frames, but just Import* or Offscreen

A case just those don't cover is deferred offscreen rendering. Currently if you want to render some RenderElements into an offscreen texture, you have to do it before the main framebuffer render. But it's desirable to render offscreens during the main framebuffer render in some cases:

• To avoid rendering offscreens that won't end up drawn (i.e. fully obscured). As I understand, ideally during render list construction you'd still damage track inside the offscreen to update its commit counter, but you wouldn't actually render the offscreen contents until the offscreen itself is drawn.
• To let a framebuffer effect composite into the offscreen, i.e. in capture_framebuffer() capture the framebuffer then draw it as a background inside the offscreen, then draw the offscreen itself.
  You can't do this currently because in draw() you only have a GlesFrame with no way to switch to a different target (the offscreen texture), so you can't call RenderElement::draw() on the offscreen contents.

Subframes let you do this. Idk if they are the simplest way to do it. Maybe some way to switch the frame's target temporarily? Though I guess that also kind of goes against what a frame is supposed to represent.

[Drakulix]

New approach! (Needs a bit of cleanup for CI, but compiles with standard feature sets.)

[Drakulix]

Based on #1816 because that made the whole mess of the new MultiRenderer-code somewhat easier to reason about.

This is ready for testing & review. :)

[cmeissl]

I am not 100% sure this should really require a new abstraction instead of generalizing it.
But I also have no better idea and the approach is clean, so imo this is good solution for now.