[AD] Implementation of draw_memory_bitmap_region

["Peter Hull" <peterhull90@xxxxxxxxxx>]

I've implemented this function for OS X using OpenGL. Does it look OK?
Maybe it could be moved to ogl_display.c if it is correct.
The function decode_allegro_format gets the appropriate OpenGL
format/size for an Allegro mode. I don't know if there's an official
way to do that.

Pete

static void draw_memory_bitmap_region(ALLEGRO_DISPLAY *display,
ALLEGRO_BITMAP *bmp,
			        float sx, float sy, float sw, float sh, float dx, float dy,
int flags) {
   int fmt, size;
   ALLEGRO_LOCKED_REGION region;
   int l = MAX((int) sx, bmp->cl);
   int r = MIN((int) (sx+sw) - 1, bmp->cr);
   int t = MAX((int) sy, bmp->ct);
   int b = MIN((int) (sy+sh) - 1, bmp->cb);
   if (l>=r || t>=b) {
      /* Clipped out */
      return;
   }
   al_lock_bitmap_region(bmp, l, t, r - l + 1, b - t + 1, &region,
ALLEGRO_LOCK_READONLY);
   decode_allegro_format(region.format,&fmt,&size,NULL);
   glPixelStorei(GL_UNPACK_ROW_LENGTH, region.pitch /
al_get_pixel_size(region.format));
   glRasterPos2f(dx - sx + l, dy - sy + t);
	glDrawPixels(bmp->lock_w, bmp->lock_h, fmt, size, region.data);
   al_unlock_bitmap(bmp);
}