swscale: Factorize FAST_BGR2YV12 definition.

It is used only for x86 so it will eventually be moved with that code
into the x86 arch subdirectory.

libswscale/rgb2rgb.c

@@ -30,8 +30,6 @@
 #include "swscale.h"
 #include "swscale_internal.h"
 
-#define FAST_BGR2YV12 // use 7-bit instead of 15-bit coefficients
-
 void (*rgb24tobgr32)(const uint8_t *src, uint8_t *dst, long src_size);
 void (*rgb24tobgr16)(const uint8_t *src, uint8_t *dst, long src_size);
 void (*rgb24tobgr15)(const uint8_t *src, uint8_t *dst, long src_size);

libswscale/swscale.c

@@ -76,8 +76,6 @@ untested special converters
 //#undef ARCH_X86
 #define DITHER1XBPP
 
-#define FAST_BGR2YV12 // use 7 bit coefficients instead of 15 bit
-
 #define isPacked(x)         (       \
            (x)==PIX_FMT_PAL8        \
         || (x)==PIX_FMT_YUYV422     \

libswscale/swscale_internal.h

@@ -31,6 +31,8 @@
 
 #define STR(s)         AV_TOSTRING(s) //AV_STRINGIFY is too long
 
+#define FAST_BGR2YV12 //use 7-bit instead of 15-bit coefficients
+
 #define MAX_FILTER_SIZE 256
 
 #if ARCH_X86