render: Add SDL_RenderDebugText().

Fixes #11201.

examples/renderer/18-debug-text/README.txt

@@ -0,0 +1,4 @@
+This example creates an SDL window and renderer, and draws some text
+using SDL_RenderDebugText(). This is not quality text rendering, but it can
+be helpful for simple apps, debugging, or showing something in a pinch.
+