Implement blargg ntsc filters

- this implements blargg's nes ntsc filters using core options
- an optional height doubling is also added but disabled for performance reasons (might make that optional as a core option)
- since PS2 and PSP have their own blitter branches, these platforms do not have the ntsc filters since i dont have the means to test on those systems.
- compile with HAVE_NTSC=1 to have these options, HAVE_NTSC=0 disabled filter including core options
- HAVE_NTSC=1 is set as default, other than PS2 and PSP as stated above.

src/ntsc/nes_ntsc_config.h

@@ -0,0 +1,27 @@
+/* Configure library by modifying this file */
+
+#ifndef NES_NTSC_CONFIG_H
+#define NES_NTSC_CONFIG_H
+
+/* Uncomment to enable emphasis support and use a 512 color palette instead
+of the base 64 color palette. */
+#define NES_NTSC_EMPHASIS 1
+
+/* The following affect the built-in blitter only; a custom blitter can
+handle things however it wants. */
+
+/* Bits per pixel of output. Can be 15, 16, 32, or 24 (same as 32). */
+#define NES_NTSC_OUT_DEPTH 16
+
+/* Type of input pixel values. You'll probably use unsigned short
+if you enable emphasis above. */
+#define NES_NTSC_IN_T unsigned char
+
+/* Each raw pixel input value is passed through this. You might want to mask
+the pixel index if you use the high bits as flags, etc. */
+#define NES_NTSC_ADJ_IN( in ) (in & 0x3F)
+
+/* For each pixel, this is the basic operation:
+output_color = color_palette [NES_NTSC_ADJ_IN( NES_NTSC_IN_T )] */
+
+#endif