rwops: Make read and write work like POSIX, not stdio.

This simplifies some things, clarifies some things, and also allows
for the possibility of RWops that offer non-blocking i/o (although
none of the current built-in ones do, intentionally, we could add this
later if we choose, or people could provide things like network socket
RWops implementations now, etc.

Fixes #6729.

test/testoverlay2.c

@@ -420,7 +420,7 @@ int main(int argc, char **argv)
         quit(2);
     }
 
-    SDL_RWread(handle, RawMooseData, MOOSEFRAME_SIZE, MOOSEFRAMES_COUNT);
+    SDL_RWread(handle, RawMooseData, MOOSEFRAME_SIZE * MOOSEFRAMES_COUNT);
 
     SDL_RWclose(handle);