Request add ncnn::Mat support for Visual Studio Image Watch Tools (#625)

* add image watch plugin for image watch tools of visual studio

* add a new blank line as the end line

tools/plugin/ImageWatchNCNN.natvis

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+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+  <UIVisualizer ServiceId="{A452AFEA-3DF6-46BB-9177-C0B08F318025}" Id="1" MenuName="Add to Image Watch"/>
+
+  <!-- Tencent NCNN ncnn::Mat support -->
+
+  <Type Name="ncnn::Mat">
+    <UIVisualizer ServiceId="{A452AFEA-3DF6-46BB-9177-C0B08F318025}" Id="1" />
+  </Type>
+
+  <Type Name="ncnn::Mat">
+    <DisplayString Condition="elemsize==4">{{FLOAT32, {c} x {w} x {h}}}</DisplayString>
+    <DisplayString Condition="elemsize==2">{{FLOAT16, {c} x {w} x {h}}}</DisplayString>
+    <DisplayString Condition="elemsize==1">{{INT8, {c} x {w} x {h}}}</DisplayString>
+    <Expand>
+      <Synthetic Name="[type]" Condition="elemsize==4">
+        <DisplayString>FLOAT32</DisplayString>
+      </Synthetic>
+      <Synthetic Name="[type]" Condition="elemsize==2">
+        <DisplayString>FLOAT16</DisplayString>
+      </Synthetic>
+      <Synthetic Name="[type]" Condition="elemsize==1">
+        <DisplayString>INT8</DisplayString>
+      </Synthetic>
+      <Item Name="[channels]">c</Item>
+      <Item Name="[width]">w</Item>
+      <Item Name="[height]">h</Item>
+      <Item Name="[planes]">c</Item>
+      <Item Name="[data]" Condition="elemsize==4">((float*)(data))</Item>
+      <Item Name="[data]" Condition="elemsize==2">data</Item>
+      <Item Name="[data]" Condition="elemsize==1">data</Item>
+      <Item Name="[stride]" Condition="elemsize==1">w</Item>
+      <Item Name="[stride]" Condition="elemsize==2">w*2</Item>
+      <Item Name="[stride]" Condition="elemsize==4">w*4</Item>
+    </Expand>
+  </Type>
+</AutoVisualizer>

tools/plugin/README.md

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+
+## NCNN Image Watch Plugin for Visual Studio
+Image Watch plugin is a good tool for better understanding insight of images. This tiny work offer a ".natvis" file which could add ncnn::Mat class support for Image Watch, and users could debug ncnn::Mat image just like debuging cv::Mat via Image Watch.
+
+To use this plugin, please move this "ImageWatchNCNN.natvis" file to "C:/user/${your user name}/Documents/Visual Studio ${VS_Version}/Visualizers" folder. If not exist this folder, create it(such as: "C:\Users\nihui\Documents\Visual Studio 2017\Visualizers"). 
+
+![](https://github.com/Tencent/ncnn/blob/master/tools/plugin/snapshot.png)
+
+See [Image Watch Help](https://imagewatch.azurewebsites.net/ImageWatchHelp/ImageWatchHelp.htm) page for more advanced using tips of Image Watch(For example, get single channel from channels, such as getting confidence heatmap from forward result list {confidence, x1, y1, x2, y2}).