Enhance OpenGlControlBase.cs

[heku]

What does the pull request do?

1. Dispose IGlContext correctly
2. Refactor OpenGL initialization check

What is the current behavior?

In the following case, the ctx is not disposed.

if(_resources == null)
{
    var contextFactory = AvaloniaLocator.Current.GetRequiredService<IPlatformGraphicsOpenGlContextFactory>();
    ctx = contextFactory.CreateContext(null);
    if (ctx.TryGetFeature<IGlContextExternalObjectsFeature>(out var externalObjects))
         // If line below returns null, ctx is not disposed.
        _resources = OpenGlControlBaseResources.TryCreate(ctx, surface, interop, externalObjects);  
    else
        ctx.Dispose();
}

if(_resources == null)
{
    Logger.TryGet(LogEventLevel.Error, "OpenGL")?.Log("OpenGlControlBase",
        "Unable to initialize OpenGL: current platform does not support multithreaded context sharing and shared memory");
    return false;
}

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051868-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]