change build system to CMake and use SDL3 for everything

libs/SDL3/examples/renderer/17-read-pixels/README.txt

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+This example creates an SDL window and renderer, and draws a
+rotating texture to it, reads back the rendered pixels, converts them to
+black and white, and then draws the converted image to a corner of the
+screen.
+
+This isn't necessarily an efficient thing to do--in real life one might
+want to do this sort of thing with a render target--but it's just a visual
+example of how to use SDL_RenderReadPixels().
+
+A better, but less visual, use of SDL_RenderReadPixels() is to make
+screenshots: you grab the current contents of the screen, and save the pixels
+as a bitmap file or whatever.