Move overscan after rendering the NTSC frame, add ntsc scanlines

- do overscan cropping after the ntsc frame is generated. this allows
removing left and right screen artifacts when using horizontal crop like
what you would see happen in TV when adjusting horizontal size (HSIZE).

- add core option that implements NTSC scanline effects. This requires
framebuffer to double the height, so a define is added (NTSC_SCANLINES at
the top of libretro.c) to disable this on some low-memory platforms or just disable NTSC filters altogether.

- some minor cleanups

src/drivers/libretro/libretro_core_options.h

@@ -92,6 +92,19 @@ struct retro_core_option_definition option_defs_common[] = {
       },
       "disabled"
    },
+#ifdef NTSC_SCANLINES
+   {
+      "fceumm_ntsc_scanlines",
+      "NTSC Scanlines",
+      "Enable scanlines when using NTSC filters.",
+      {
+         { "disabled", NULL },
+         { "enabled",  NULL },
+         { NULL, NULL },
+      },
+      "disabled"
+   },
+#endif /* NTSC_SCANLINES */
 #endif
    {
       "fceumm_palette",