OpenGL interop: Need a way to purge VAO FBO cache.

[WangHoi]

When initializing the Engine by Attaching to Existing OpenGL/GLES Context.
And working with mutiple GL contexts.

We need a way to purge engine's internal VAO and FBO cache of one specific GL context,
when that context is about to be destroyed.

[TheMostDiligent]

But why do you need to explicitly purge it? The caches will be purged when the device is destroyed.

[WangHoi]

The OpenGL contexts recreated when window got resized，new float preview window opens, etc.
Recreate diligent RenderDevice is very slow, need to load and initialize resources again and again.
We are using shared OpenGL context to workaround it.
Current remain problem is VAO FBO cache memory leaking.

[WangHoi]

I also think add the PurgeCaches param to DeviceContextGLImpl::UpdateCurrentGLContext is wrong.
Because a new current OpenGL context is active now, and Diligent may call glDeleteVertexArrays() in new context with old context's VAOs.

RenderDeviceGLImpl::PurgeContextCaches with old OpenGL context activated is the right way.

---

The following steps works:
Comment out the VERIFY check in the dtors of VAOCache and FBOCache,
During context re-creation, call RenderDeviceGLImpl::PurgeContextCaches before the old context is about to be released.

[WangHoi]

I'm experimenting with the following local modifications:

Added a IDeviceContextGL::PurgeCurrentContextCaches() .

// Implements `IDeviceContextGL::PurgeCurrentContextCaches()`
void DeviceContextGLImpl::PurgeCurrentContextCaches()
{
    auto NativeGLContext = m_pDevice->m_GLContext.GetCurrentNativeGLContext();
    if (NativeGLContext != NULL)
        m_pDevice->PurgeContextCaches(NativeGLContext);
}

And instead of commenting the VERIFY checks in VAO/FBOCache's destructors, add purge methods to them.

[TheMostDiligent]

Can you send the PR with your changes?

[WangHoi]

Sent a PR here:
DiligentGraphics/DiligentCore#629

[TheMostDiligent]

Thank you! I will take a look

[TheMostDiligent]

I renamed PurgeCurrentContextCaches to PurgeCurrentGLContextCaches for consistency with UpdateCurrentGLContext

[WangHoi]

😄 Thank you it works now.