Added rencache invalidation on window-exposed event

Fixes #63

src/api/system.c

···
 #include <errno.h>
 #include <sys/stat.h>
 #include "api.h"
+#include "rencache.h"
 #ifdef _WIN32
   #include <windows.h>
 #endif
···
         lua_pushnumber(L, e.window.data2);
         return 3;
       } else if (e.window.event == SDL_WINDOWEVENT_EXPOSED) {
-        SDL_UpdateWindowSurface(window);
+        rencache_invalidate();
+        lua_pushstring(L, "exposed");
+        return 1;
       }
       /* on some systems, when alt-tabbing to the window SDL will queue up
       ** several KEYDOWN events for the `tab` key; we flush all keydown

src/rencache.c

···
 }
 
 
+void rencache_invalidate(void) {
+  memset(cells_prev, 0xff, sizeof(cells_buf1));
+}
+
+
 void rencache_begin_frame(void) {
   /* reset all cells if the screen width/height has changed */
   int w, h;
···
   if (screen_rect.width != w || h != screen_rect.height) {
     screen_rect.width = w;
     screen_rect.height = h;
-    memset(cells_prev, 0xff, sizeof(cells_buf1));
+    rencache_invalidate();
   }
 }
 

src/rencache.h

···
 void rencache_set_clip_rect(RenRect rect);
 void rencache_draw_rect(RenRect rect, RenColor color);
 int  rencache_draw_text(RenFont *font, const char *text, int x, int y, RenColor color);
+void rencache_invalidate(void);
 void rencache_begin_frame(void);
 void rencache_end_frame(void);