Add Shape::Callback to do custom rendering inside of an egui UI

[emilk]

This adds a new shape Shape::Callback which can be used to inline custom rendering code inside an egui UI. The callback will be given the underlying rendering context, and will therefore be integration specific. When using eframe, it will be glow::Context (unless one opts in to the glium or WebGL backends).

This means you can paint anything anywhere, including behind or on top of other egui windows and with transparency. Previously this was only possible by first rendering to a texture and then displaying that texture using egui::TextureId.

The design is very similar to Dear ImGui's AddCallback/UserCallback.

Closes #1198

Unresolved issues

One unresolved issue is where to free any resources (shaders, buffer, textures, …) allocated by the custom painting callback. Perhaps we could pass a Rc<Cell<glow::Context>> and let the user clone it and destroy the resources on Drop?

[coderedart]

glow::Context doesn't need to be mutable. so, a simple Rc<glow::Context> should be good. window must be alive for Rc<glow::Context> to work in their User struct Drop impl (otherwise, segfaults). .

one question i have is whether the user will be able to access the textures loaded into egui already? I would assume they don't want to duplicate textures in their app used both by egui::Image as well as their own custom rendering code, so reusing the textures from egui's tex manager would be cool.

[emilk]

one question i have is whether the user will be able to access the textures loaded into egui already? I would assume they don't want to duplicate textures in their app used both by egui::Image as well as their own custom rendering code, so reusing the textures from egui's tex manager would be cool.

Great question!

My initial thinking was that a user would either want to either manage their textures manually OR ask egui to do so, but not both. But I guess we could pass the full egui_glow::Painter along with glow::Context so the users could ask for any textures they desire.

[KentaTheBugMaker]

if we able to redirecting to Texture would save energy.

in this case pass Painter to user would better.

[emilk]

There - now the callback code has access to egui_glow::Painter which contains the glow::Context and all textures.

[emilk]

I think this is a decent start.

Future work: Make glow a first-class part of the epi/eframe interface so that users can run glow-related setup- and teardown code. Coming in:

• eframe app creation refactor #1363