need to update sdl

external/SDL/examples/renderer/17-read-pixels/README.txt

@@ -1,12 +0,0 @@
-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.