Use PNG files for tests and examples

These are much smaller than the previous BMP files

Fixes https://github.com/libsdl-org/SDL/issues/14159

examples/renderer/06-textures/README.txt

@@ -1,3 +1,3 @@
 This example creates an SDL window and renderer, loads a texture from a
-.bmp file, and then draws it a few times each frame.
+.png file, and then draws it a few times each frame.