Fix `GL_INVALID_OPERATION` if closing a tab via a "Do you want to save changes?" popup

[adamkewley]

Debugging OpenGL via WSL2 with this repro:

• Boot osc
• Ensure OpenGL debugging is enabled (About > Debug Mode)
• Open (e.g.) the model editor
• Ensure you make a change that will prompt "Do you want to save changes" (required)
• Click No
• The console/log will show an OpenGL error (below) and return to the splash screen

Error message:

id = 1
message = GL_INVALID_OPERATION in glBindTexture(non-gen name)
source = GL_DEBUG_SOURCE_API
type = GL_DEBUG_TYPE_ERROR
severity = GL_DEBUG_SEVERITY_HIGH

The reason that this happens is because ImGui's texture renderer does not manage the lifetime of the texture. So the sequence of events is something like:

• Something in the tab emits a 3D render as a texture to ImGui
• The texture ends up in ImGui's render queue
• Then the user clicks No in the save dialog
• Then the osc::Tab is destructed, killing the texture
• Then the ImGui queue is rendered by ImGui
• Which is invalid, because ImGui's render queue now contains a texture reference that should be dead
• 💥

The solution is to either:

• Have osc::Texture2Ds and osc::RenderTextures stored in a datastructure that sits next to ImGui, and is cleared only after ImGui draws its render queue
• (hacky, but will work) ensure the top-level tabbed UI screen only ever destroys an osc::Tab after ImGui has flushed its internal render queue

[adamkewley]

This bug has been in OSC for a couple of years now so it's non-fatal and non-blocking, but I'm documenting it here as something that is also related to the fact that ImGui is using a separate rendering backend from OSC (osc uses oscar's renderer whereas ImGui uses its own one that's packaged with ImGui).

The "hollistic" approach here is to write an oscar backend to ImGui, so that all GPU interactions go through one managed graphics API.

[adamkewley]

This is now fixed because OSC now uses its own ImGui backend, which includes lifetime management for any submitted texture handles.