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/README.md

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+gif-h
+=====
+
+This one-header library offers a simple, very limited way to create animated GIFs directly in code.
+Those looking for particular cleverness are likely to be disappointed; it's pretty much a straight-ahead
+implementation of the GIF format with optional Floyd-Steinberg dithering. (It does at least use delta
+encoding - only the changed portions of each frame are saved.) 
+
+So resulting files are often quite large. The hope is that it will be handy nonetheless as a quick and easily-integrated way for programs to spit out animations.
+
+Only RGBA8 is currently supported as an input format. (The alpha is ignored.) 
+
+Email me : ctangora -at- gmail -dot- com
+
+Usage:
+-------------------
+Create a GifWriter struct. 
+
+Pass the struct to GifBegin() to initialize values and write the file header.
+
+Pass frames of the animation to GifWriteFrame().
+
+Finally, call GifEnd() to close the file handle and free memory.
+
+	#include <vector>
+	#include <cstdint>
+	#include <gif.h>
+	int main()
+	{
+		int width = 100;
+		int height = 200;
+		std::vector<uint8_t> black(width * height * 4, 0);
+		std::vector<uint8_t> white(width * height * 4, 255);
+
+		auto fileName = "bwgif.gif";
+		int delay = 100;
+		GifWriter g;
+		GifBegin(&g, fileName, width, height, delay);
+		GifWriteFrame(&g, black.data(), width, height, delay);
+		GifWriteFrame(&g, white.data(), width, height, delay);
+		GifEnd(&g);
+
+		return 0;
+	}
+