Only read first entity snapshot from GOTV demos, fix position jittering

[saul]

Position jittering fixed by skipping over m_vecX/Y/Z that are entirely 0 bits (i.e. default(float)). Fixes #27

Improves GOTV parsing time by ~10%

Method	Job	Arguments	Mean	Error	StdDev	Ratio	Gen0	Gen1	Gen2	Allocated	Alloc Ratio
ParseDemo	Job-IQMOBV	/p:Baseline=true	1.513 s	0.0094 s	0.0079 s	1.00	104000.0000	19000.0000	9000.0000	703.21 MB	1.00
ParseDemo	Job-TKWDSX	Default	1.373 s	0.0216 s	0.0202 s	0.90	84000.0000	4000.0000	2000.0000	603.5 MB	0.86

[github-actions]

BenchmarkDotNet v0.13.9+228a464e8be6c580ad9408e98f18813f6407fb5a, Ubuntu 22.04.3 LTS (Jammy Jellyfish)
AMD EPYC 7763, 1 CPU, 4 logical and 2 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 7.0.14 (7.0.1423.51910), X64 RyuJIT AVX2
  Job-WPEVXF : .NET 7.0.14 (7.0.1423.51910), X64 RyuJIT AVX2
  Job-VMPLNL : .NET 7.0.14 (7.0.1423.51910), X64 RyuJIT AVX2

InvocationCount=1  MaxIterationCount=16  UnrollFactor=1  
WarmupCount=1  

Method	Job	Arguments	Mean	Error	StdDev	Ratio	Gen0	Gen1	Allocated	Alloc Ratio
ParseDemo	Job-WPEVXF	/p:Baseline=true	2.655 s	0.0168 s	0.0157 s	1.00	7000.0000	1000.0000	681.38 MB	1.00
ParseDemo	Job-VMPLNL	Default	2.419 s	0.0063 s	0.0053 s	0.91	6000.0000	1000.0000	583.88 MB	0.86