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/15-cliprect/README.txt

@@ -1,5 +1,5 @@
 This example creates an SDL window and renderer, loads a texture
-from a .bmp file, and stretches it across the window. Each frame, we move
+from a .png file, and stretches it across the window. Each frame, we move
 the clipping rectangle around, so only a small square of the texture is
 actually drawn.