GPU: Pseudo-implement horizontal scaling.

It's not really known how this actually works. Some testing has shown that this probably performs no filtering, and common usage in games suggests it's not actually resizing the image at all.
However, this patch does seem to fix some homebrew showing quasi-duplicated images while still keeping other applications in a working state.

src/core/hw/gpu.h

@@ -157,6 +157,9 @@ struct Regs {
             BitField< 8, 3, PixelFormat> input_format;
             BitField<12, 3, PixelFormat> output_format;
             BitField<16, 1, u32> output_tiled;     // stores output in a tiled format
+
+            // TODO: Not really sure if this actually scales, or even resizes at all.
+            BitField<24, 1, u32> scale_horizontally;
         };
 
         INSERT_PADDING_WORDS(0x1);