Add gif-h helper to create screen capture

Press key `I` (shift+`i`) to start a screen capture creating a gif with
the captured screen. Hit `I` again to stop the screen recording.

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Add gif-h helper header only library

Copied from https://github.com/charlietangora/gif-h

3d2657b9ad

gif-h/gif-h-demo.cpp

@@ -0,0 +1,80 @@
+//
+// gif-h-demo.cpp
+// by Charlie Tangora
+// Public domain.
+// Email me : ctangora -at- gmail -dot- com
+//
+// Shows an example usage of gif.h
+//
+
+#include "gif.h"
+
+#include <math.h>
+
+const int width = 256;
+const int height = 256;
+uint8_t image[ width * height * 4 ];
+
+void SetPixel( int xx, int yy, uint8_t red, uint8_t grn, uint8_t blu )
+{
+    uint8_t* pixel = &image[(yy*width+xx)*4];
+    pixel[0] = red;
+    pixel[1] = blu;
+    pixel[2] = grn;
+    pixel[3] = 255;  // no alpha for this demo
+}
+
+void SetPixelFloat( int xx, int yy, float fred, float fgrn, float fblu )
+{
+    // convert float to unorm
+    uint8_t red = (uint8_t)roundf( 255.0f * fred );
+    uint8_t grn = (uint8_t)roundf( 255.0f * fgrn );
+    uint8_t blu = (uint8_t)roundf( 255.0f * fblu );
+    
+    SetPixel( xx, yy, red, grn, blu );
+}
+
+int main(int argc, const char * argv[])
+{
+    const char* filename = "./MyGif.gif";
+    if( argc > 1 )
+    {
+        filename = argv[1];
+    }
+    
+    // Create a gif
+    GifWriter writer = {};
+    GifBegin( &writer, filename, width, height, 2, 8, true );
+    
+    for( int frame=0; frame<256; ++frame )
+    {
+        
+        // Make an image, somehow
+        // this is the default shadertoy - credit to shadertoy.com
+        float tt = frame * 3.14159f * 2 / 255.0f;
+        for( int yy=0; yy<height; ++yy )
+        {
+            for( int xx=0; xx<width; ++xx )
+            {
+                float fx = xx / (float)width;
+                float fy = yy / (float)height;
+                
+                float red = 0.5f + 0.5f * cosf(tt+fx);
+                float grn = 0.5f + 0.5f * cosf(tt+fy+2.f);
+                float blu = 0.5f + 0.5f * cosf(tt+fx+4.f);
+                
+                SetPixelFloat( xx, yy, red, grn, blu );
+            }
+        }
+        
+        
+        // Write the frame to the gif
+        printf( "Writing frame %d...\n", frame );
+        GifWriteFrame( &writer, image, width, height, 2, 8, true );
+    }
+    
+    // Write EOF
+    GifEnd( &writer );
+    
+    return 0;
+}