Reduce allocations in GraphicsDevice.PlatformApplyRenderTargets()

Specialized the ArrayEqualityComparer to avoid allocations in EqualityComparer<T>.Equals(). GetHashCode() is also slightly faster.
Those methods are called each time GraphicsDevice.glFramebuffer.TryGetValue() is called in PlatformApplyRenderTargets() meaning it is called each frame and can result in quite a bit of memory allocations.