Parse XPM without SDL_image

We encounter some problems with SDL2_image on MSYS2 (Windows), so
implement our own XPM parsing which does not depend on SDL_image.

The input XPM is considered safe (it's in our source repo), so do not
check XPM format errors. This implies that read_xpm() is not safe to
call on any unsafe input.

Although less straightforward, use SDL_CreateRGBSurfaceFrom() instead of
SDL_CreateRGBSurfaceWithFormatFrom() because it is available with SDL
versions older than 2.0.5.

app/src/tinyxpm.h

@@ -0,0 +1,3 @@
+#include <SDL2/SDL.h>
+
+SDL_Surface *read_xpm(char *xpm[]);