Fix `ShowFPS` of `D2DControl` and some performance improvement

This PR fixes a missing assignment of lastFpsUpdate in D2DControl and also improves performance of showing the fps by using DateTime.UtcNow instead of DateTime.Now.

Some benchmarks to show the performance improvements


BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19043.1387 (21H1/May2021Update)
Intel Core i7-6700 CPU 3.40GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
.NET SDK=6.0.100
  [Host]     : .NET 5.0.12 (5.0.1221.52207), X64 RyuJIT
  Job-FPINJZ : .NET 5.0.12 (5.0.1221.52207), X64 RyuJIT
  Job-IQQFOL : .NET 6.0.0 (6.0.21.52210), X64 RyuJIT
  Job-OFJKZZ : .NET Framework 4.8 (4.8.4420.0), X64 RyuJIT

Force=True

Method	Runtime	Runs	Mean	Error	StdDev	Ratio	Allocated
Now	.NET 5.0	1	204.26 ns	1.312 ns	1.227 ns	1.00	-
UtcNow	.NET 5.0	1	74.39 ns	0.697 ns	0.618 ns	0.36	-

Now	.NET 6.0	1	136.24 ns	2.503 ns	2.782 ns	1.00	-
UtcNow	.NET 6.0	1	26.20 ns	0.295 ns	0.261 ns	0.19	-

Now	.NET Framework 4.8	1	265.86 ns	5.322 ns	5.915 ns	1.00	-
UtcNow	.NET Framework 4.8	1	67.75 ns	1.379 ns	1.588 ns	0.26	-

Now	.NET 5.0	100	20,321.98 ns	182.166 ns	170.399 ns	1.00	-
UtcNow	.NET 5.0	100	7,436.71 ns	80.009 ns	74.841 ns	0.37	-

Now	.NET 6.0	100	11,260.58 ns	209.422 ns	241.171 ns	1.00	-
UtcNow	.NET 6.0	100	2,719.37 ns	49.646 ns	46.439 ns	0.24	-

Now	.NET Framework 4.8	100	26,292.46 ns	284.250 ns	251.980 ns	1.00	-
UtcNow	.NET Framework 4.8	100	6,745.38 ns	133.738 ns	125.098 ns	0.26	-

[Params(1, 100)]
public int Runs { get; set; }

[Benchmark(Baseline = true)]
public void Now()
{
    for (int i = 0; i < Runs; i++)
    {
        _ = System.DateTime.Now;
    }
}

[Benchmark]
public void UtcNow()
{
    for (int i = 0; i < Runs; i++)
    {
        _ = System.DateTime.UtcNow;
    }
}

jingwood / d2dlib

Fix `ShowFPS` of `D2DControl` and some performance improvement #77

Some benchmarks to show the performance improvements