Introduce Context.WithLock

[dizzyd]

Summary

• Introduces Context.WithLock() method to provide thread-safe access to context operations
• Wraps texture loading and freeing queue operations with proper mutex protection
• Prevents race conditions when multiple threads enqueue texture operations simultaneously

The texture loading and freeing queues were not thread-safe when accessed from multiple goroutines. This could lead to queue corruption and unpredictable behavior when textures were being loaded or freed concurrently.

Thus, I added a WithLock() method to the GIUContext that safely executes functions while holding the context mutex. This ensures that:

1. Texture load requests enqueued via EnqueueNewTextureFromRgba() are thread-safe
2. Texture free requests triggered by finalizers are thread-safe
3. Queue processing in the render loop is protected from concurrent modifications

Files modified:

• Context.go: Added WithLock() method for safe mutex-protected operations
• MasterWindow.go: Wrapped texture queue processing in WithLock() during render
• Texture.go: Protected queue operations in both EnqueueNewTextureFromRgba() and the texture finalizer